Tibial bearing component for a knee prosthesis with improved articular characteristics

ABSTRACT

An orthopaedic knee prosthesis includes a tibial bearing component with articular features which operate to protect adjacent soft tissues of the natural knee, promote and/or accommodate desired articulation with an abutting femoral component, and facilitate expedient and effective implantation by a surgeon.

BACKGROUND 1. Technical Field

The present disclosure relates to orthopaedic prostheses and,specifically, to articular tibial components in a knee prosthesis.

2. Description of the Related Art

Orthopaedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For a damaged knee, a kneeprosthesis may be implanted using a tibial baseplate, a tibial bearingcomponent, and a distal femoral component. The tibial baseplate isaffixed to a proximal end of the patient's tibia, which is typicallyresected to accept the baseplate. The femoral component is implanted ona distal end of the patient's femur, which is also typically resected toaccept the femoral component. The tibial bearing component is placedbetween the tibial baseplate and femoral component, and may be fixedupon or slidably coupled to the tibial baseplate.

The tibial bearing component, which may also be referred to as a tibialinsert or meniscal component, provides an articular surface whichinteracts with the adjacent femur or femoral component during extensionand flexion of the knee. The features and geometry of the articularsurface influences the articular characteristics of the knee, such as bydefining maximum knee flexion, internal/external rotation, femoralrollback, and behavior of the knee prosthesis in hyperextension, forexample. Accordingly, substantial design efforts have previously focusedon providing knee prosthesis components which preserve flexion range andpromote a desired kinematic motion profile for the widest possible rangeof prospective knee replacement patients.

SUMMARY

The present disclosure provides an orthopaedic knee prosthesis includinga tibial bearing component with articular features which operate toprotect adjacent soft tissues of the natural knee, promote and/oraccommodate desired articulation with an abutting femoral component, andfacilitate expedient and effective implantation by a surgeon.

Features which accommodate and protect soft tissues of the kneeinclude 1) a relief or scallop formed in the proximal peripheral edge ofthe bearing component near an anterior/lateral corner thereof; and 2) abulbous, convex flare protruding from the tibial bearing componentsidewall at an anterior/medial portion thereof.

Features which facilitate and/or promote improved articularcharacteristics include: 1) medial and lateral articular tracks, definedby respective dished articular compartments of the tibial bearingcomponent, which are angled or “clocked” with respect to the posterioredge of the tibial bearing component; 2) a lateral articular compartmentwhich defines a low conformity with the corresponding condyle of theabutting femoral component, and a medial articular compartment whichdefines a high conformity with the corresponding medial condyle of thefemoral component; 3) medial and lateral articular tracks which, whenviewed in respective sagittal planes, define a distal-most point whichis anteriorly shifted with respect to predicate devices; 4) a lateralarticular track which transitions from an early- and mid-flexion paththat is generally linear along an anterior/posterior path as viewed in atransverse plane, to an arcuate path at the deep-flexion, posterior endof the articular track; 5) a lateral articular compartment which definesa relatively “flattened” posterior edge profile as compared to theposterior edge profile of the medial articular compartment to define adifferential “jump height” therebetween; 6) for posterior-stabilized(PS) prostheses, a spine defining a posterior face which transitionsfrom symmetrical in a proximal portion (i.e., a portion contacted by afemoral cam in early flexion) to an angled configuration in a distalportion (i.e., a portion contacted by the femoral cam in mid- to deepflexion); and 7) for ultra-congruent (UC) knee prostheses, a posterioreminence disposed between medial and lateral articular compartments thatis sized and shaped to smoothly transition into a position within theintercondylar notch of an abutting femoral component when the kneeprosthesis is hyperextended.

Features which facilitate surgical implantation include provision offamilies of tibial bearing components from which the surgeon may chooseintraoperatively. These families may include a range of component sizes,multiple components within a given size, and different componentdesigns. For example, within a range of sizes, different components mayfeature varying clocking angles and/or levels of posterior “flattening”in the lateral articular compartment, as noted above. Within a givensize, multiple components may feature differing thickness profiles, asviewed from a sagittal and/or coronal perspective, in order toselectively tilt or cant the articular surface. Moreover, variouscombinations of the design features described herein may be providedacross several tibial bearing component designs, such asposterior-stabilized, ultra-congruent and cruciate-retaining designs.

According to one embodiment thereof, the present invention provides atibial bearing component for articulation with a medial femoral condyleand a lateral femoral condyle, the tibial bearing component defining atibial bearing component coordinate system comprising: a bearingcomponent transverse plane extending along a medial/lateral directionand an anterior/posterior direction; a bearing component coronal planeextending along a proximal/distal direction and the medial/lateraldirection, the bearing component coronal plane perpendicular to thebearing component transverse plane; and a bearing component sagittalplane extending along the anterior/posterior direction and theproximal/distal direction, the bearing component sagittal planeperpendicular to the bearing component transverse plane and the bearingcomponent coronal plane, the tibial bearing component comprising: anarticular surface and an opposing distal surface, the distal surfaceparallel to the bearing component transverse plane, the articularsurface including medial and lateral dished articular compartments sizedand shaped for articulation with the medial and lateral femoral condylesrespectively, the medial and lateral dished articular compartmentsseparated from one another by the bearing component sagittal plane, thelateral articular compartment comprising a plurality of coronalcross-sectional profiles defining a lateral set of coronal distal-mostpoints spanning a lateral anterior/posterior extent, the lateral set ofcoronal distal-most points defining a lateral articular track, thelateral articular track having an anterior portion and a posteriorportion, the anterior portion defining a nominally straight line whenprojected onto the bearing component transverse plane, the posteriorportion defining a curved line when projected onto the bearing componenttransverse plane.

According to another embodiment thereof, the present invention providesa tibial bearing component for articulation with a medial femoralcondyle and a lateral femoral condyle, the tibial bearing componentdefining a tibial bearing component coordinate system comprising: abearing component transverse plane extending along a medial/lateraldirection and an anterior/posterior direction; a bearing componentcoronal plane extending along a proximal/distal direction and themedial/lateral direction, the bearing component coronal planeperpendicular to the bearing component transverse plane; and a bearingcomponent sagittal plane extending along the anterior/posteriordirection and the proximal/distal direction, the bearing componentsagittal plane perpendicular to the bearing component transverse planeand the bearing component coronal plane, the tibial bearing componentcomprising: an articular surface and an opposing distal surface, thedistal surface parallel to the bearing component transverse plane, thearticular surface including medial and lateral dished articularcompartments sized and shaped for articulation with the medial andlateral femoral condyles respectively, the medial and lateral dishedarticular compartments separated from one another by the bearingcomponent sagittal plane, the articular and distal surfaces bounded by atibial bearing periphery, the lateral articular compartment comprising aplurality of coronal cross-sectional profiles defining a lateral set ofcoronal distal-most points spanning a lateral anterior/posterior extent,the lateral set of coronal distal-most points defining a lateralarticular track having an anterior portion and a posterior portion, theanterior portion defining a nominally straight line when projected ontothe bearing component transverse plane, the anterior portion of thelateral articular track extrapolated posteriorly to define a lateralintersection point with the tibial bearing periphery, the medialarticular compartment comprising a plurality of coronal cross-sectionalprofiles defining a medial set of coronal distal-most points spanning amedial anterior/posterior extent, the medial set of coronal distal-mostpoints defining a medial articular track, the medial articular trackdefining a nominally straight line when projected onto the bearingcomponent transverse plane, the medial articular track extrapolatedposteriorly to define a medial intersection point with the tibialbearing periphery, the lateral and medial intersection points joined bya posterior line of the tibial bearing component, at least one of thelateral articular track and the medial articular track defining an acuteangle with the posterior line.

According to yet another embodiment thereof, the present inventionprovides a family of tibial bearing components for articulation withfemoral condyles, each of the family of tibial bearing componentsdefining a tibial bearing component coordinate system comprising: abearing component transverse plane extending along a medial/lateraldirection and an anterior/posterior direction; a bearing componentcoronal plane extending along a proximal/distal direction and themedial/lateral direction, the bearing component coronal planeperpendicular to the bearing component transverse plane; and a bearingcomponent sagittal plane extending along the anterior/posteriordirection and the proximal/distal direction, the bearing componentsagittal plane perpendicular to the bearing component transverse planeand the bearing component coronal plane, the family of tibial bearingcomponents comprising a small tibial bearing component and a largetibial bearing component, the small and large tibial bearing componentseach comprising: an articular surface and an opposing distal surface,the distal surface parallel to the bearing component transverse plane,the articular surface including medial and lateral dished articularcompartments sized and shaped for articulation with the femoralcondyles, the medial and lateral dished articular compartments separatedfrom one another by the bearing component sagittal plane, the articularand distal surfaces bounded by a tibial bearing periphery, the lateralarticular compartment comprising a plurality of coronal cross-sectionalprofiles defining a lateral set of coronal distal-most points spanning alateral anterior/posterior extent, the lateral set of coronaldistal-most points defining a lateral articular track having an anteriorportion and a posterior portion, the anterior portion defining anominally straight line when projected onto the bearing componenttransverse plane, the anterior portion of the lateral articular trackextrapolated posteriorly to define a lateral intersection point with thetibial bearing periphery, the medial articular compartment comprising aplurality of coronal cross-sectional profiles defining a medial set ofcoronal distal-most points spanning a medial anterior/posterior extent,the medial set of coronal distal-most points defining a medial articulartrack, the medial articular track defining a nominally straight linewhen projected onto the bearing component transverse plane, the medialarticular track extrapolated posteriorly to define a medial intersectionpoint with the tibial bearing periphery, the lateral and medialintersection points joined by a posterior line, at least one of thelateral articular track and the medial articular track defining an acuteangle with the posterior line; and the acute angle of the small tibialbearing component less than the acute angle of the large tibial bearingcomponent.

According to still another embodiment thereof, the present inventionprovides a tibial bearing component for articulation with a medialfemoral condyle and a lateral femoral condyle, the tibial bearingcomponent defining a tibial bearing component coordinate systemcomprising: a bearing component transverse plane extending along amedial/lateral direction and an anterior/posterior direction; a bearingcomponent coronal plane extending along a proximal/distal direction andthe medial/lateral direction, the bearing component coronal planeperpendicular to the bearing component transverse plane; and a bearingcomponent sagittal plane extending along the anterior/posteriordirection and the proximal/distal direction, the bearing componentsagittal plane perpendicular to the bearing component transverse planeand the bearing component coronal plane, the tibial bearing componentcomprising: an articular surface and an opposing distal surface, thedistal surface parallel to the bearing component transverse plane, thearticular surface including medial and lateral dished articularcompartments sized and shaped for articulation with the medial andlateral femoral condyles respectively, the medial and lateral dishedarticular compartments separated from one another by the bearingcomponent sagittal plane, the articular and distal surfaces bounded by atibial bearing periphery, the lateral articular compartment comprising aplurality of coronal cross-sectional profiles defining a lateral set ofcoronal distal-most points spanning a lateral anterior/posterior extent,the lateral set of coronal distal-most points defining a lateralarticular track having an anterior portion and a posterior portion, themedial articular compartment comprising a plurality of coronalcross-sectional profiles defining a medial set of coronal distal-mostpoints spanning a medial anterior/posterior extent, the medial set ofcoronal distal-most points defining a medial articular track; and meansfor clocking the medial articular track and the lateral articular trackinto a counterclockwise clocked rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1A is a top plan view of a posterior stabilized (PS) tibial bearingcomponent and baseplate in accordance with the present disclosure;

FIG. 1B is a graph plotting the angular arrangement of articular tracksof various sizes of ultra-congruent tibial bearing components inaccordance with the present disclosure;

FIG. 1C is a graph plotting the angular arrangement of articular tracksof various sizes of posterior-stabilized tibial bearing components inaccordance with the present disclosure:

FIG. 1D is a graph plotting the angular arrangement of articular tracksof various sizes of cruciate-retaining tibial bearing components inaccordance with the present disclosure;

FIG. 2 is a perspective view of a femoral component in accordance withthe present disclosure:

FIG. 3A is a sagittal, cross-sectional view of a tibial bearingcomponent in accordance with the present disclosure, taken through amedial articular compartment along line 3A-3A of FIG. 1A;

FIG. 3B is a sagittal, cross-sectional view of a tibial bearingcomponent in accordance with the present disclosure, taken through alateral articular compartment along line 3B-3B of FIG. 1A;

FIG. 3C is a graph plotting the height differential between medial andlateral posterior compartment edges for various sizes ofposterior-stabilized tibial bearing components in accordance with thepresent disclosure;

FIG. 3D is a graph plotting the height differential between medial andlateral posterior compartment edges for various sizes of ultra-congruenttibial bearing components in accordance with the present disclosure;

FIG. 3E is a graph plotting the anterior/posterior position of medialdistal-most points of an articular surface for tibial bearing componentsin accordance with the present disclosure and prior art tibial bearingcomponents (where prior art devices are listed as “predicate”);

FIG. 3F is a graph plotting the anterior/posterior position of lateraldistal-most points of an articular surface for tibial bearing componentsin accordance with the present disclosure and prior art tibial bearingcomponents (where prior art devices are listed as “predicate”);

FIG. 4A is an elevation, cross-sectional view of the tibial bearingshown in FIG. 1A, together with a femoral component made in accordancewith the present disclosure, taken in a coronal plane;

FIG. 4B is an elevation, cross-sectional view of the tibial bearing andfemoral components shown in FIG. 4A, taken in a sagittal plane throughthe lateral articular condyle and articular compartment thereof;

FIG. 4C is an elevation, cross-sectional view of the tibial bearing andfemoral components shown in FIG. 4A, taken in a sagittal plane throughthe medial articular condyle and articular compartment thereof;

FIG. 5A is a top perspective view of the tibial bearing component shownin FIG. 1A;

FIG. 5B is a sagittal, cross-sectional view of the tibial bearingcomponent shown in FIG. 5A, taken along the line 5B-5B of FIG. 5A;

FIG. 5C is another sagittal, cross-sectional view of the tibial bearingcomponent shown in FIG. 5A, taken along the line 5C-5C of FIG. 5A:

FIG. 5D is another sagittal, cross-sectional view of the tibial bearingcomponent shown in FIG. 5A, taken along the line 5D-5D of FIG. 5A;

FIG. 6A is a top plan view of an ultracongruent (UC) tibial bearingcomponent made in accordance with the present disclosure;

FIG. 6B is a perspective view of the tibial bearing component shown inFIG. 6A, shown positioned atop a tibial baseplate;

FIG. 6C is an elevation, cross-sectional view of the tibial bearingcomponent shown in FIG. 6A, taken in a coronal plane;

FIG. 6D is a sagittal, elevation, cross-sectional view of the tibialbearing component of FIG. 6A, in combination with a femoral component;

FIG. 6E is a fragmentary, anterior perspective view of a prior artultracongruent (UC) tibial bearing component, illustrating a posterioreminence thereof (where prior art devices are listed as “predicate”);

FIG. 7A is a top, perspective view of a cruciate-retaining (CR) tibialbearing component made in accordance with the present disclosure;

FIG. 7B is a top plan view of the tibial bearing component shown in FIG.7A;

FIG. 8A is a side, elevation view of another ultracongruent (UC) tibialbearing component in accordance with the present disclosure,illustrating an anterior medial bulbous flare;

FIG. 8B is a bottom plan view of the tibial bearing component show inFIG. 8A;

FIG. 9A is a sagittal, cross-sectional view of a tibial bearingcomponent in accordance with the present disclosure, illustratinggeometric changes to the distal surface of the tibial bearing componentwhich affect the anterior/posterior orientation of the tibial articularsurfaces with respect to the tibia;

FIG. 9B is a sagittal, cross-sectional view of the tibial bearingcomponent of FIG. 9A, in which the geometric changes to the tibialbearing component replicate a decrease in the anteroposterior slopedefined by the resected surface of the tibia;

FIG. 9C is a sagittal, cross-sectional view of the tibial bearingcomponent of FIG. 9A, in which the geometric changes to the tibialbearing component replicate an increase in the anteroposterior slopedefined by the resected surface of the tibia;

FIG. 9D is a sagittal, cross-sectional view of a tibial bearingcomponent in accordance with the present disclosure, illustratinggeometric changes to the articular surface of the tibial bearingcomponent which affect the anterior/posterior orientation of the tibialarticular surfaces with respect to the tibia;

FIG. 10A is a coronal, cross-sectional view of a tibial bearingcomponent in accordance with the present disclosure, illustratingpotential geometric changes to the distal surface of the tibial bearingcomponent which affect the medial/lateral orientation of the tibialarticular surfaces with respect to the tibia;

FIG. 10B is a coronal, cross-sectional view of an alternative tibialbearing component, in which one of the potential geometric changes tothe bearing component shown in FIG. 10A is effected to compensate for avalgus deformity;

FIG. 10C is a coronal, cross-sectional view of an alternative tibialbearing component, in which one of the potential geometric changes tothe bearing component shown in FIG. 10A is effected to compensate for avarus deformity; and

FIG. 11 is a perspective, exploded view illustrating assembly of atibial bearing component and tibial baseplate made in accordance withthe present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure provides tibial bearing components for a kneeprosthesis in which the bearing components have various features whichenhance articular characteristics throughout a range of motion whilealso protecting the soft tissues of the knee after implantation.

In order to prepare the tibia and femur for receipt of a knee jointprosthesis of the present disclosure, any suitable methods orapparatuses for preparation of the knee joint may be used. Exemplarysurgical procedures and associated surgical instruments are disclosed in“Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique”, “NEXGENCOMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed BearingKnee” and “Zimmer NexGen Complete Knee SolutionExtramedullary/Intramedullary Tibial Resector, Surgical Technique”(collectively, the “Zimmer Surgical Techniques”), the entireties ofwhich are hereby expressly incorporated herein by reference, copies ofwhich are filed in an information disclosure statement on even dateherewith.

As used herein. “proximal” refers to a direction generally toward thetorso of a patient, and “distal” refers to the opposite direction ofproximal, i.e., away from the torso of a patient. “Anterior” refers to adirection generally toward the front of a patient or knee, and“posterior” refers to the opposite direction of anterior, i.e., towardthe back of the patient or knee. In the context of a prosthesis alone,such directions correspond to the orientation of the prosthesis afterimplantation, such that a proximal portion of the prosthesis is thatportion which will ordinarily be closest to the torso of the patient,the anterior portion closest to the front of the patient's knee, etc.

Similarly, knee prostheses in accordance with the present disclosure maybe referred to in the context of a coordinate system includingtransverse, coronal and sagittal planes of the component. Uponimplantation of the prosthesis and with a patient in a standingposition, a transverse plane of the knee prosthesis is generallyparallel to an anatomic transverse plane, i.e., the transverse plane ofthe knee prosthesis is inclusive of imaginary vectors extending alongmedial/lateral and anterior/posterior directions. However, in someinstances the bearing component transverse plane may be slightly angledwith respect to the anatomic transverse plane, such as when the proximalsurface of the resected tibia T (FIGS. 3A and 3B) definesanteroposterior slope S (described in detail below). In FIGS. 3A and 3B,tibia T is shown with a positive anteroposterior slope, in that theproximal resected surface of tibia T is not normal to anatomic axisA_(T) of tibia T. Where such anteroposterior slope S is non-zero, thebearing component transverse plane will be angled with respect to theanatomic transverse plane, with the magnitude of such angle beingapproximately equal to the magnitude of the anteroposterior slope S.

Coronal and sagittal planes of the knee prosthesis are also generallyparallel to the coronal and sagittal anatomic planes in a similarfashion. Thus, a coronal plane of the prosthesis is inclusive of vectorsextending along proximal/distal and medial/lateral directions, and asagittal plane is inclusive of vectors extending alonganterior/posterior and proximal/distal directions. As with therelationship between the anatomic and bearing component transverseplanes discussed above, it is appreciated that small angles may beformed between the bearing component sagittal and coronal planes and thecorresponding anatomic sagittal and coronal planes depending upon thesurgical implantation method. For example, creation of anteroposteriorslope S (FIGS. 3A and 3B) will angle the bearing component coronal planewith respect to the anatomic coronal plane, while alteration of theresected surface S for correction of a varus or valgus deformity willangle the bearing component sagittal plane with respect to the anatomicsagittal plane.

As with anatomic planes, the sagittal, coronal and transverse planesdefined by the knee prosthesis are mutually perpendicular to oneanother. For purposes of the present disclosure, reference to sagittal,coronal and transverse planes is with respect to the present kneeprosthesis unless otherwise specified.

The embodiments shown and described herein illustrate components for aleft knee prosthesis. Right and left knee prosthesis configurations aremirror images of one another about a sagittal plane. Thus, it will beappreciated that the aspects of the prosthesis described herein areequally applicable to a left or right knee configuration.

A tibial bearing component made in accordance with the presentdisclosure provides an articular surface with features and geometrywhich promote and accommodate an articular profile similar to a healthynatural knee. As described in detail below, features incorporated intothe tibial bearing component articular surface advantageously provide anoptimal level of constraint and motion guidance throughout a wide rangeof knee flexion.

Prosthesis designs in accordance with the present disclosure may includeposterior stabilized (PS) prostheses and mid level constraint (MLC)prostheses, each of which includes spine 38 (FIG. 1A) and femoral cam 40(FIG. 2) designed to cooperate with one another to stabilize femoralcomponent 20 with respect to tibial bearing component 12 in lieu of aresected posterior cruciate ligament (PCL). For purposes of the presentdisclosure, PS and MLC prostheses are both of a “posterior-stabilized”design, which includes spine 38 extending proximally from the articularsurface, in which the spine is spaced posteriorly from an anterior edgeof the periphery of tibial bearing component 12 (FIG. 1A). Spine 38 isdisposed between medial and lateral dished articular compartments 16,18.

Another contemplated design includes “cruciate retaining” (CR)prostheses, such as those using components configured as shown in FIGS.4A and 4B. CR designs omit spine 38 and femoral cam 40, such thatfemoral component 220 defines an intercondylar space between medial andlateral condyles 222, 224 that is entirely open and uninterrupted byfemoral cam 40. CR tibial components are generally used in surgicalprocedures which retain the PCL. Cruciate-retaining (CR) type tibialbearing component 212 is illustrated in FIGS. 7A and 7B. Tibial bearingcomponent 212 and femoral component 220 are substantially similar totibial bearing component 12 and femoral component 20 described herein,respectively, with reference numerals of components 212, 220 analogousto the reference numerals used in component 12, 20 except with 200 addedthereto. Structures of tibial bearing component 212 and femoralcomponent 220 correspond to similar structures denoted by correspondingreference numerals of tibial bearing component 12 and femoral component20, except as otherwise noted.

Referring to FIG. 7A, posterior cutout 236 is sized and positioned toaccommodate a posterior cruciate ligament upon implantation of tibialbearing component 212. Intercompartmental eminence 238 comprises anintercondylar ridge disposed between medial and lateral articularcompartments 216, 218 and extending anteroposteriorly from posterior 236cutout to anterior relief space 261. Thus, the intercondylar ridgedefined by intercompartmental eminence 238 is disposed between themedial and lateral dished articular compartments and occupies theavailable anterior/posterior space therebetween.

Anterior relief space 261 is also disposed generally between medial andlateral articular compartments 216, 218, anterior of intercondylareminence 238, and extending posteriorly from an anterior edge of theperiphery of tibial bearing component 212. An exemplary embodiment ofrelief space 261 is described in U.S. Provisional Patent ApplicationSer. No. ______ (Attorney Docket No. ZIM0912-03), entitled TIBIALBEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS and filed on even date herewith, the entire disclosureof which is hereby expressly incorporated herein by reference.

Yet another design includes “ultra congruent” (UC) prostheses, shown inFIGS. 6A, 6B, 8A and 8B, which also omits spine 38 and femoral cam 40but is designed for use with a patient whose PCL is resected. Referringto FIGS. 6A and 6B, for example, ultra-congruent tibial bearingcomponent 112 is illustrated which includes posterior eminence 138.Posterior eminence 138 extends proximally from the articular surface oftibial bearing component 112, by a distance more than intercondylareminence 238 and less than spine 38. Posterior eminence 138 also extendsanteriorly from a posterior edge of the tibial bearing periphery, in thearea normally occupied by posterior cutout 36 (FIG. 1A). Thus, posterioreminence 138 is distinguished from spine 38 in that posterior eminence138 resides at the posterior edge of tibial bearing component 112, andin that it defines an intermediate height above the surroundingarticular surface. Like spine 38 and intercompartmental eminence 238,posterior eminence 138 is disposed between the medial and lateral dishedarticular compartments 116, 118.

“Congruence.” in the context of knee prostheses, refers to thesimilarity of curvature between the convex femoral condyles and thecorrespondingly concave tibial articular compartments. A detaileddiscussion of congruence appears below. UC designs utilize very highcongruence between the tibial bearing compartments and femoral condylesto provide prosthesis stability, particularly with respect toanterior/posterior relative motion.

In the exemplary embodiments described below, tibial bearing components12, 112, 212 are each adapted to fixedly attach to tibial baseplate 14,such that the resulting tibial prosthesis is a “fixed-bearing” design.For purposes of illustration, tibial bearing component 212 is shown inFIG. 11. As shown in FIG. 11, distal surface 260 of tibial bearingcomponent 212 includes a two-pronged recess 280 which cooperates with acorrespondingly shaped two-prong boss 80 protruding proximally from tray84 of tibial baseplate 14. Further, a peripheral undercut 282 formedaround the periphery of distal surface 260 of tibial bearing component212 is sized and shaped to receive peripheral wall 82. Upon assembly,tibial bearing component 212 is advanced along path P, such that tibialbearing component moves along a generally anterior-to-posterior path asrecess 280 begins to engage with boss 80. Further posterior movement oftibial bearing component 212 causes a tight interfitting engagementbetween recess 280 and boss 80, and eventually aligns peripheralundercut 282 with peripheral wall 82. When so aligned, tibial bearingcomponent 212 “snaps” into fixed engagement with tibial baseplate 14.Posterior-stabilized tibial bearing component 12 and ultra-congruenttibial bearing component 112 may fixedly engage with tibial baseplate ina similar fashion.

Once such fixed engagement takes place, tibial bearing component 212 (orcomponents 12 or 112) is immovable with respect to tibial baseplate 14.As used herein, a “fixed bearing” tibial prosthesis is a prosthesis inwhich a bearing component is seated atop a tibial baseplate in a final,locked position such as the arrangement described above. In this lockedposition, lift-off of bearing components 12, 112, 212 from tibialbaseplate 14, as well as transverse movement of bearing components 12,112, 212 relative to tibial baseplate 14, is prevented during naturalarticulation of the knee. While some very small amount of motion(sometimes referred to as micromotion) may occur between tibial bearingcomponents 12, 112, 212 and tibial baseplate 14 in a fixed bearingprosthesis, no such motion occurs by design along a designated path.

Exemplary fixed-bearing securement designs are described in U.S. PatentApplication Publication No. 2012/0035737, filed Jul. 22, 2011 andentitled TIBIAL PROSTHESIS (Attorney Docket No. ZIM0806-02), and in U.S.Patent Application No. 2012/0035735, filed Jul. 22, 2011 and entitledTIBIAL PROSTHESIS (Attorney Docket No. ZIM0806-03), the entiredisclosures of which are hereby expressly incorporated herein byreference. Other types of fixed bearing prostheses include “monoblock”type designs, in which the tibial bearing component is permanentlymolded over the tibial baseplate to create a unitary tibial prosthesis.However, it is also contemplated that the features of a tibial bearingcomponent described herein may be used on a “mobile bearing” prosthesisdesign in which the tibial bearing component is allowed to move relativeto the tibial baseplate during articulation.

Except as otherwise specified herein, all features described below maybe used with any potential prosthesis design. While a particular designmay potentially include all the features described herein, it iscontemplated that some prosthesis designs may include selected featuresdescribed herein but omit other such features, as required or desiredfor a particular application.

1. Articular Tracks: Arcuate Posterior/Lateral Bearing Path for DeepFlexion Rollback

FIG. 1A illustrates tibial prosthesis 10 having tibial bearing component12 and tibial baseplate 14. The perspective of FIG. 1A is atransverse-plane view of tibial prosthesis 10, looking down upon theproximally facing articular surface of bearing component 12, such thatdistal surface 60 (FIG. 3A) is substantially parallel to the transverseplane. Bearing component 12 includes medial articular compartment 16 andlateral articular compartment 18, each defining concave dished articularsurfaces sized and shaped to articulate with femoral condyles, e.g.,prosthetic condyles such as medial and lateral condyles 22, 24 offemoral component 20 (FIG. 2). For purposes of the present disclosure, acentral sagittal plane may be said to bisect tibial prosthesis 10 into amedial portion including medial articular compartment 16 and a lateralportion including lateral compartment 18.

During articulation from knee extension to flexion, the contact pointbetween condyles 22, 24 and articular compartments 16, 18 movesposteriorly, thereby defining medial articular track 26 and lateralarticular track 28, respectively. Articular tracks 26, 28 are alsorepresentative of the lowest points along the anterior/posterior extentof medial and lateral articular compartments 16, 18. More particularly,any given coronal cross-section of articular compartments 16, 18 (suchas, for example, the coronal cross-section shown in FIG. 4A) definesmedial and lateral distal-most points in medial and lateral articularcompartments 16, 18, respectively. These distal-most points are eachcoincident with medial and lateral articular tracks 26, 28,respectively. When the distal-most points of all possible coronalcross-sections (i.e., every coronal cross-section across the entireanterior/posterior extent of medial and lateral articular compartments16, 18) are aggregated, the set of distal-most points form lines whichdefine medial and lateral articular tracks 26, 28 respectively. Asdescribed in detail below, the location of distal-most points 42, 44 ofarticular compartments 16, 18 may be determined accounting for orignoring the anteroposterior tibial slope S (FIGS. 3A and 3B), it beingunderstood that the magnitude of slope S influences theanterior/posterior positions of distal-most points 42, 44. It iscontemplated that either method of determining the locations ofdistal-most points 42, 44 may be appropriate in some instances, while inother instances a particular method is appropriate. For purposes of thepresent disclosure, both methods of determining the anterior/posteriorpositions of distal-most points 42, 44 may be used except whereotherwise specified.

For convenience, the present discussion refers to “points” or “lines” ofcontact between tibial bearing component 12 and femoral component 20along articular tracks 26, 28. However, it is of course appreciated thateach potential point or line of contact (i.e., any of the points alongone of articular tracks 26, 28) is not truly a point or line, but ratheran area of contact. These areas of contact may be relatively larger orsmaller depending on various factors, such as prosthesis materials, theamount of pressure applied at the interface between tibial bearingcomponent 12 and femoral component 20, and the like. Moreover, it isappreciated that some of the factors affecting the size of the contactarea may change dynamically during prosthesis use, such as the amount ofapplied pressure at the femoral/tibial interface during walking,climbing stairs or crouching, for example. For purposes of the presentdiscussion, a “contact point” may be taken as the point at the geometriccenter of the area of contact. The “geometric center”, in turn, refersto the intersection of all straight lines that divide a given area intotwo parts of equal moment about each respective line. Stated anotherway, a geometric center may be said to be the “average” (i.e.,arithmetic mean) of all points of the given area. Similarly, a “contactline” is the central line of contact passing through and bisecting anelongate area of contact.

Referring still to FIG. 1A, medial articular track 26 defines agenerally straight line extending along an anterior/posterior directionwhen viewed from above (i.e., when projected onto the transverse plane)as shown in FIG. 1A. Thus, as medial condyle 22 of femoral component 20articulates with medial compartment 16 of tibial bearing component 12,the point of contact therebetween follows a generally straightanterior/posterior path as projected onto the transverse plane. Forpurposes of the present disclosure, a “straight” line or path defined bya component of a knee prosthesis refers to a nominally straight line orpath, it being appreciated that manufacturing tolerances andcircumstances of in vivo use may cause such straight lines or paths todeviate slightly from the nominal path. As used herein, a “nominal”quantity or feature refers to a feature as designed, notwithstandingvariabilities arising from manufacturing and/or use.

On the other hand, lateral articular track 28 includes arcuate portion30 near the posterior edge of lateral articular compartment 18. Thecontact point between lateral condyle 24 and lateral articularcompartment 18 follows a generally straight-line anteroposterior paththroughout early and mid flexion, such that an anterior portion oflateral articular track 28 is linear in a similar fashion to medialarticular track 26. However, when prosthesis 10 reaches a deep flexionconfiguration and the contact point between lateral condyle 24 andlateral articular compartment 18 advances toward the posterior portionof lateral compartment 18, the corresponding posterior portion ofarticular track 28 curves or arcs inwardly to define a curved lineforming arcuate portion 30.

In the exemplary embodiment of FIG. 1A, arcuate portion 30 of articulartrack 28 defines an arc having a radius R_(T) defining radius centerC_(T), which is spaced medially from lateral articular track 28. In theillustrative embodiment of FIG. 1A, this medial spacing is equal to themedial/lateral separation distance D_(T) (FIG. 1A) between the parallellinear portions of medial and lateral articular tracks 26, 28, such thatradius center C_(T) of radius R_(T) is coincident with medial articulartrack 26. Radius Rr may be between as little as 30 mm, 34 mm or 36 mmand as large as 48 mm, 52 mm or 60 mm, or may be any size within anyrange defined by any of the foregoing values. The magnitude of Radius Rrgenerally grows larger as the size of tibial bearing component 12increases across a range of prosthesis sizes.

In addition to the coronal distal-most points described above, each ofmedial and lateral articular tracks 26, 28 include an arcuate sagittalprofile (shown in FIGS. 3A and 3B and described below) defining sagittaldistal-most points 42, 44 respectively. Referring to FIG. 1A, theanterior/posterior position of radius center C_(T) is, in an exemplaryembodiment, coincident with distal-most point 42 thereof as viewed inthe transverse plane perspective of FIG. 1A. Further discussion ofdistal-most point 42 appears below within the context of an implantedknee prosthesis. For purposes of the illustration of FIG. 1A, however,distal-most point 42 may be taken to be the point in lateral compartment18 which is closest to distal surface 60 of tibial bearing component 12(see FIG. 4B).

In addition, arcuate portion 30 defines a point of tangency with thelinear anterior remainder of articular track 28 at transition point 31,such that transition point 31 represents the posterior terminus of suchlinear anterior portion and the anterior terminus of arcuate portion 30of articular track 28. In the exemplary embodiment of FIG. 1A, radiuscenter C_(T) and transition point 31 of lateral articular track 28 liein a common coronal plane. Stated another way, the linear/arcuatetransition point 31 of lateral articular track 28 and radius centerC_(T) of medial articular track 26 share a common anteroposteriorlocation along their respective articular tracks 26, 28.

Advantageously, setting the magnitude of radius R_(T) equal to bearingspacing distance D_(T) accommodates external rotation of the femur,which causes femoral component 20 (FIG. 2) to pivot in deep flexionabout the contact point between medial condyle 22 and medial articularcompartment 16. This contact point is coincident with radius centerC_(T), such that lateral condyle 24 follows the path of least resistanceupon lateral articular compartment 18 even as external rotation and theassociated femoral rollback occurs.

In an exemplary embodiment, arcuate portion 30 of lateral articulartrack 28 occupies as little as 20% or 25% and as much as 28%, 35% or 50%of the overall anterior/posterior extent of lateral articularcompartment 18, or may occupy any percentage within any range defined byany of the foregoing values. This anterior/posterior location oftransition point 31 cooperates with the articular surface geometry oflateral articular compartment 18 and the articular surface geometry oflateral condyle 24 of femoral component 20 to set the initial level offlexion for engagement of condyle 24 with arcuate portion 30 ofarticular track 28 at approximately 90 degrees of flexion, though it isappreciated that the actual initial engagement may vary substantiallydepending on, for example, unique patient anatomy and the particularconditions of articulation during prosthesis use.

As noted above, it is contemplated that articular tracks 26, 28 asdescribed herein may be incorporated into ultra-congruent,posterior-stabilized and cruciate-retaining designs, and that thebenefits and advantages conferred by the disclosed arrangement ofarticular tracks 26, 28 may be realized in any knee prosthesis design.

2. Articular Tracks: Rotational Orientation with Respect to PosteriorEdge of the Tibial Prosthesis.

Articular tracks 26, 28 are angled with respect to the posterior edgesof tibial bearing component 12 and tibial baseplate 14, which promotes asimilarly angled orientation of articular track 26, 28 upon implantationto facilitate enhanced prosthesis articulation. Such angling may bedefined in the context of tibial bearing component 12 alone, asdescribed below, and/or when tibial bearing component 12 is attached totibial baseplate 14.

Referring still to FIG. 1A, tibial bearing component 12 defines an acuteangle α between posterior line 32 (described in detail below) and medialarticular track 26. Because medial articular track 26 and the linearanterior portion of lateral articular track 28 arc parallel to oneanother (as noted above), angle α is also defined between the linearanterior portion of lateral articular track 28 and posterior line 32.

Similarly, angle θ is defined between posterior line 34 of tibialbaseplate 14 and articular tracks 26, 28. As described in detail below,the medial compartment of tibial baseplate 14 extends furtherposteriorly compared to the posterior/medial edge of tibial bearingcomponent 12, but tibial bearing component 12 and tibial baseplate 14define similar anteroposterior extents in their respective lateralsides. Therefore, as shown in FIG. 1A, angle θ is less than angle α.

To form posterior lines 32, 34 as shown in FIG. 1A, medial articulartrack 26 and the linear anterior portion of lateral articular track 28are first extrapolated posteriorly to intersect with the outerperipheries defined by tibial bearing component 12 and tibial baseplate14, respectively. Posterior line 32 of tibial bearing component 12 isthen defined as the line which joins medial and lateral intersectionpoints P_(TM), P_(TL) between medial and lateral articular tracks 26, 28and the periphery of tibial bearing component 12. Posterior line 34 oftibial baseplate 14 is the line which joins intersection points P_(BM),P_(BL) between medial and lateral articular tracks 26, 28 and theperiphery of tibial baseplate 14.

In an exemplary embodiment, angle α defined by tibial bearing component12 alone may be only slightly less than 90 degrees, such as by 0.5degrees. In other embodiments and across various prosthesis sizes, angleα may be less than 90 degrees by as much as 9 degrees or more. Forexample, referring to FIG. 1B, angle α for various sizes ofcruciate-retaining prosthesis designs are illustrated, with sizes 1 and7 (on the horizontal axis) being the smallest and largest componentsizes, respectively, and the intermediate sizes 2-6 growingprogressively in size. For such cruciate-retaining designs, angle αranges from 81 degrees to 89.5 degrees across the sevencruciate-retaining component sizes.

Referring to FIG. 1C, angle α for seven sizes (again shown on thehorizontal axis) is illustrated for an ultra-congruent prosthesisdesign. Angle α, as shown on the vertical axis, ranges from 82 degreesto 88.7 degrees across the seven ultra-congruent component sizes.

Referring to FIG. 1D, angle α for eleven sizes of posterior-stabilizedprosthesis designs are illustrated, with sizes 1 and 11 (on thehorizontal axis) being the smallest and largest component sizes,respectively, and the intermediate sizes 2-10 growing progressively insize. Angle α, again on the vertical axis, ranges from 81.7 degrees to86.7 degrees across the eleven posterior-stabilized component sizes.

FIGS. 1B-1D all illustrate a family of tibial bearing components withina given design class (i.e., posterior-stabilized, ultra-congruent orcruciate-retaining), in which each family exhibits an upward trend inangle α as the prosthesis size grows larger. Generally speaking, angle αexperiences a minimum value for the smallest component size and alargest value for the largest component size, with angle α inintermediate component sizes following an upward trend fromsmallest-to-largest. In some instances, the next-largest size willdefine a decreased angle α as compared to the next-smallest size, asillustrated in FIGS. 1B-1D. However, a substantial majority of sizesexperience an increase in angle α from smaller to larger sizes, as wellas the overall substantial increase exhibited by the overall change fromthe smallest to largest size. Therefore, it may be said that the trendin angle α is generally upward across the range of sizes.

Angle 9 is less than angle α, and deviates from angle α by any amountgreater than 0 degrees. In an exemplary embodiment, angle θ is less thanangle α by as little as 0.01 degrees, 0.4 degrees or 1 degree and aslarge as 6 degrees, 8.8 degrees or 15 degrees, or may be any valuewithin any range defined by any of the foregoing values. The differencebetween angle θ and angle α generally smaller for small prosthesis sizesand larger for large prosthesis sizes.

Advantageously, the rotation of articular tracks 26, 28 with respect toposterior lines 32, 34 rotates or “clocks” tibial bearing component 12into a counterclockwise orientation, as viewed from above, as comparedto a non-rotated or centered orientation (in which angles α and/or βwould be 90-degrees). Stated another way, such “clocking” can be thoughtof as rotation of the proximal, articular surface of a tibial bearingcomponent while leaving the distal, baseplate-contacting surfacenon-rotated. Clocking in accordance with the present disclosure istherefore analogous to disconnecting articular compartments 16, 18 fromdistal surface 60, rotating articular compartments 16, 18 in acounterclockwise direction (as viewed from above), and reconnectingarticular compartments 16, 18 to distal surface 60 in the new, rotatedorientation. In this regard, the structure and arrangement of tibialbearing component 12 provides means for clocking articular tracks 26,28.

Such clocking yields an improved articular profile which more closelymimics natural motion of the knee, reduces wear of the prosthesiscomponents, and enhances prosthesis longevity. More particularly, tibialbearing component 12 promotes clinically successful prosthesis functionby providing a correct orientation and position of the tibiofemoral“bearing couple” with respect to one another. The bearing couple iscomprised of femoral component 20 and tibial bearing component 12. Inprosthesis 10, articular compartments 16, 18 are fixed to tibialbaseplate 14 and therefore the tibial component defines the articularsurface orientation with respect to tibia T (see, e.g., FIG. 3A).Femoral component 20, which is mounted to the distal end of the femur F,is not mechanically coupled to tibial bearing component 12, but insteadarticulates therewith along an articular profile influenced by themating articular surfaces of tibial bearing component 12 and femoralcomponent 20. Thus, the placement and articular geometry of tibialbearing component 12 helps establish the lower (distal) half of thebearing couple.

The clocking of tibial articular tracks 26, 28, in cooperation with theasymmetric periphery of tibial baseplate 14, discourages implantation oftibial bearing component 12 such that tracks 26, 28 are relativelyinternally rotated. By preventing such internal rotation of tracks 26,28, tibial bearing component 12 provides smooth cooperation with theknee's soft tissues during in vivo knee articulation by ensuring thatthe articular bearing motion is properly oriented relative to the femurto deliver desired knee kinematics, range of motion (ROM) and stability.Advantageously, this cooperation promotes decreased material wear intibial bearing component 12, enhanced prosthesis stability, proper kneebalance, and high ROM.

Further, the substantial coverage provided by tibial baseplate 14 andthe clocked orientation of articular tracks 26, 28 with respect theretoencourages proper rotation of tibial bearing component 12 uponimplantation. When a bone-contacting surface of a properly sized tibialbaseplate 14 is mated with a resected tibia, the asymmetric peripherythereof results in substantial coverage of the resected proximal surfaceand largely controls the rotational orientation thereof. A detaileddescription of the periphery of tibial baseplate 14 and the attendantsubstantial coverage of a resected proximal tibia is described in U.S.Patent Application Publication No. 2012/0022659 filed Jul. 22, 2011 andentitled “ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS” (AttorneyDocket No. ZIM0815-01), the entire disclosure of which is herebyexpressly incorporated by reference herein. With tibial baseplate 14properly oriented, fixing tibial bearing component 12 thereto will setthe location and orientation of bearing component 12, which will then beautomatically “clocked” in the advantageous manner described above.

The amount of rotation or “clocking” of articular tracks 26, 28 may varydepending on prosthesis design and/or prosthesis size (as describedabove). For any given prosthesis design in a particular style and for aparticular sized tibia, it is contemplated that a second tibial bearingcomponent 12 may be provided which defines a different magnitude ofclocking but is otherwise identical to the first tibial bearingcomponent 12. Thus, two tibial bearing components 12 useable with acommon tibial baseplate 14 and femoral component 20—but each withdifferent levels of clocking—may be provided and chosen by a surgeonpreoperatively or intraoperatively. Similarly, a set of three or moretibial bearing components 12 may be provided, each sharing a common sizeand prosthesis design, but all having different levels of clocking.

3. Articular Tracks: Anterior Shift of Bearing Compartment Distal-MostPoints.

Referring now to FIGS. 3A and 3B, medial and lateral articularcompartments 16, 18 define distal-most points 42, 44, respectively.Distal-most points 42, 44 are coincident with medial and lateralarticular tracks 26, 28, respectively, and represent the distal-mostpoints from a sagittal perspective on articular tracks 26, 28 whentibial bearing component 12 is implanted upon tibia T with ananteroposterior slope S of 5 degrees. Tibial baseplate 14, having aconstant thickness across its anterior/posterior extent, does not affectthe value of anteroposterior slope S. Anteroposterior slope S referencesa zero degree slope line 46, which is defined by a generally transversereference plane normal to anatomic axis A_(T) of tibia T. For purposesof the present disclosure, proximal and distal directions are directionsnormal to the reference plane (and, therefore, parallel to anatomic axisA_(T) after implantation of tibial prosthesis 10).

Tibial bearing component 12 is a “high-flexion” prosthetic component, inthat the geometry and configuration of articular compartments 16, 18cooperate with a femoral component (e.g., femoral component 20 of FIGS.4A and 4B) to allow a large total range of motion. For example, ahigh-flexion knee prosthesis may enable a flexion range of as little as130 degrees, 135 degrees, or 140 degrees and as large as 150 degrees,155 degrees or 170 degrees, or may enable any level of flexion withinany range defined by any of the foregoing values. In the context ofhigh-flexion components, enablement of high flexion refers to theability of a prosthesis to reach a given level of flexion byarticulation of condyles 22, 24 with articular compartments 16, 18 andwithout impingement of any prosthesis structures with non-articularprosthesis surfaces. While tibial bearing component 12 enables highprosthesis flexion as described below, it is of course appreciated thatthe actual level of flexion achievable for any given patient is alsodependent upon various anatomical and surgical factors.

For tibial bearing component 12, high flexion may be enabled by one orboth of two features. First, tibial bearing component 12 includesdifferential heights H_(L), H_(M), with H_(L) less than H_(M) tofacilitate posterior rollback of lateral condyle 24 in deep flexion (asdescribed in detail below). For purposes of the present disclosure,heights H_(L), H_(M) are measured normal to slope line 46. When lateralcondyle 24 is allowed to roll back in this manner, potential impingementbetween the articular surface of condyle 24 and/or the adjacent femoralbone against the posterior/lateral periphery of tibial bearing component12 is avoided. Second, the medial/posterior periphery of tibial bearingcomponent 12 includes posterior chamfer surface 27 (disposed at theposterior periphery of medial articular compartment 16, as shown in FIG.3A), which slopes in a posterior direction from proximal-to-distal.Chamfer 27 creates an absence of a vertical peripheral wall immediatelyposterior of medial articular compartment 16, thereby creating acorresponding space the adjacent femoral bone and/or adjacent softtissues in deep flexion. An exemplary embodiment of posterior/medialchamfer 27 is described in detail in U.S. patent application Ser. No.13/229,103, filed Sep. 9, 2011 and entitled MOTION FACILITATING TIBIALCOMPONENT FOR A KNEE PROSTHESIS (Attorney Docket No. ZIM0815-04), theentire disclosure of which is hereby expressly incorporated herein byreference.

High flexion is also accommodated by a differential in curvature betweenmedial and lateral condyles 22, 24. For example, lateral condyle 24 offemoral component 20 may have a larger radius of curvature than medialcondyle 22 thereof. An exemplary femoral component is described in U.S.Pat. No. 6,770,099, filed Nov. 19, 2002, titled FEMORAL PROSTHESIS, theentire disclosure of which is expressly incorporated by referenceherein. During flexion and extension, the larger lateral condyle 24 offemoral component 20 tends to travel a greater distance along lateralarticular track 28 of tibial bearing component 12 as compared to thesmaller medial condyle 22 of femoral component 20. This difference indistance traveled over a given range of knee flexion may be described as“big wheel/little wheel” movement, and is a feature which enables highflexion of the knee prosthesis by encouraging advancement of lateralcondyle 24 toward the posterior edge of lateral articular compartment 18at high levels of flexion.

In tibial bearing component 12, medial and lateral distal-most points42, 44 are shifted anteriorly with respect to predicate prostheses whichenable comparably high levels of flexion, as described below. Forpurposes of the present disclosure, the relative anterior/posteriorlocation of distal-most points 42, 44 are measured by the distancesAP_(DM), AP_(DL) of distal-most points 42, 44 from the anterior edge ofthe tibial prosthesis (FIGS. 3A and 3B). For purposes of comparison,distances AP_(DM), AP_(DL) may each be expressed as a percentage of theoverall anteroposterior extent AP_(M), AP_(L) of medial and lateralprosthesis portions, which is inclusive of tibial bearing component 12and tibial baseplate 14 (FIGS. 1A, 3A and 3B) and is measured along theextrapolated articular tracks 26, 28 (as shown in FIG. 1A and describedherein). For example, if distal-most point 42 were located in the middleof overall anteroposterior extent AP_(M) of medial articular compartment16, then distal-most point 42 would be considered to be disposed at ananteroposterior location of approximately 50%. If distal-most point 42were located near the posterior edge of articular compartment 16, thendistal-most point would be near a 100% anteroposterior location.Conversely, if distal-most point 42 were located near the anterior edgeof articular compartment 16, the distal-most point 42 would be near a 0%anteroposterior location.

For purposes of the present disclosure, medial anterior/posterior extentAP_(M)(FIG. 1A) of the medial portion of tibial baseplate 14 is found byextrapolating medial articular track 26 anteriorly and posteriorly tointersect the periphery of baseplate 14 (in similar fashion to theintersection points used to define posterior line 34 described above),then measuring the distance between the resulting medial posterior andanterior intersection points. Similarly, lateral anterior/posteriorextent AP_(L) (FIG. 1A) of the lateral portion of tibial baseplate 14 isfound by extrapolating the linear anterior portion of lateral articulartrack 28 anteriorly and posteriorly to intersect the periphery ofbaseplate 14, then measuring the distance between the resulting lateralposterior and anterior intersection points.

Turning to FIG. 3E, a graphical representation of the anterior/posteriorposition of medial distal-most point 42 (FIG. 3A) is illustrated ascompared to predicate high-flexion and non-high-flexion prostheses. Intibial bearing component 12, the anterior/posterior position of medialdistal-most point 42 (FIG. 3A) is in the range of 59% to 63% whenimplanted at an anterior/posterior slope S equal to 5 degrees. Bycomparison, one prior art high-flexion device is the Zimmer Natural KneeFlex Ultracongruent Tibial Bearing Component, which places itscorresponding medial distal-most point in the range of 67% and 70% whenimplanted at a slope angle S of 5 degrees. Thus, the prior art ZimmerNatural Knee Flex Ultracongruent Tibial Bearing Component defines mediallow points which are consistently posterior of medial distal-most point42. On the other hand, the prior art Zimmer Natural Knee IIUltracongruent Tibial Bearing Component places its corresponding medialdistal-most point between 63% and 68% when implanted at a slope angle Sof 5 degrees, but the Zimmer Natural Knee II Ultracongruent TibialBearing Component does not enable high flexion at least up to 130degrees.

As for lateral compartment 18 (FIGS. 3B and 3F) of tibial bearingcomponent 12, distal-most point 44 has an anterior/posterior position ofbetween 68% and 74%. The prior art high-flexion design, the ZimmerNatural Knee Flex Ultracongruent Tibial Bearing Component mentionedabove, places such lateral distal-most points at between 70% and 73%when implanted at a slope angle S of 5 degrees. The non-high-flexionprior art design, the Zimmer Natural Knee II Ultracongruent TibialBearing Component mentioned above, places its distal-most point atbetween 66% and 70.5% when implanted at a slope angle S of S degrees.

Thus, the present ultracongruent prosthesis, as exemplified by tibialbearing component 12, blends a high-flexion design enabling at least 130degrees of knee flexion with low points that are relatively furtheranterior as compared to prior art ultracongruent prostheses.Advantageously, this anterior low-point shift discourages “paradoxicalmovement,” or movement between the femur and tibia in an oppositepattern from normal articulation. For example, the anterior shift ofdistal-most points 42, 44 inhibits anterior sliding of femoral component20 with respect to tibial bearing component 12 when the knee isarticulating from extension toward early flexion. Such early-flexionarticulation is normally accompanied by a slight posterior shift in thecontact points between condyles 22, 24 of femoral component 20 andarticular compartments 16, 18 of tibial bearing component 12. Thisposterior shift is facilitated—and a paradoxical anterior shift isinhibited—by the relative anterior positioning of distal-most points 42,44. Meanwhile, the potential of high-flexion articulation is preservedby the high-flexion features incorporated into tibial bearing component12, as described in detail herein.

The above discussion regarding anterior shift of articular surface lowpoints refers to exemplary ultracongruent (UC) type tibial bearingcomponents. However, such anterior shift may be applied to tibialbearing components of other designs, such as cruciate-retaining (CR) andposterior-stabilized (PS) designs.

4. Articular Features: Differential Conformity in Medial/LateralArticular Compartments.

Referring now to FIGS. 4A-4C, femoral component 220 and tibial bearingcomponent 212 are shown. For purposes of the following discussion,femoral component 20 and tibial bearing component 12 will be describedin the context of FIGS. 4A-4C, it being appreciated that any potentialprosthesis design (e.g., PS, UC and CR type femoral components) may eachinclude the present described features as noted above.

Femoral component 20 cooperates with tibial bearing component 12 toprovide relatively low conformity between lateral condyle 24 and lateralarticular compartment 18, and relatively high conformity between medialcondyle 22 and medial articular compartment 16.

A convex surface may be considered to be highly conforming with acorresponding concave surface where the two surfaces have similar oridentical convex and concave geometries, such that the convex surface“nests” or tightly interfits with the concave surface. For example, ahemisphere having a radius perfectly conforms (i.e., defines highconformity) with a corresponding hemispherical cavity having the sameradius. Conversely, the hemisphere would have no conformity with anadjacent flat or convex surface.

Femoral condyles 22, 24 define a coronal conformity with tibialarticular compartments 16, 18, respectively, as shown in FIG. 4A.Similarly, femoral condyles 22, 24 define sagittal conformity with thecorresponding articular compartments 16, 18, respectively, as shown inFIG. 4B. Thus, medial condyle 22 cooperates with medial articularcompartment 16 to define a medial conformity comprised of both a medialsagittal conformity and a medial coronal conformity. Similarly, lateralfemoral condyle 24 cooperates with lateral articular compartment 18 todefine a lateral conformity comprised of the lateral sagittal conformityand lateral coronal conformity. Although only a single prosthesis isshown in FIGS. 4A-4C, it is contemplated that conformity may besimilarly defined across a range of prosthesis sizes within a particularprosthesis design.

For purposes of the present disclosure, any given component ofconformity is defined as a ratio of two radii. Referring to FIG. 4A, alateral coronal conformity is defined by the ratio of the coronal radiusof lateral articular compartment 18 of tibial bearing component 12 alonglateral articular track 28, which is illustrated as radius R_(CTL)(where CTL stands for coronal, tibial, lateral) to the correspondingcoronal radius of lateral condyle 24 of femoral component 20,illustrated as radius R_(CFL) (where CFL denotes coronal, femoral,lateral). The conformity defined by R_(CTL):R_(CFL) is a number greaterthan 1, because femoral condyle 24 is designed to fit within lateralarticular compartment 18 to define point contact therewith, as describedin detail above.

Similarly, medial coronal conformity is defined by the ratioR_(CTM):R_(CFM) (where M denotes medial). Sagittal conformity betweenlateral condyle 24 and lateral articular compartment 18 is defined asthe ratio R_(STL):R_(SFL) (FIG. 4B, where S denotes sagittal, F denotesfemoral. T denotes tibia, and L denotes lateral). Medial condyle 22defines sagittal conformity with medial articular compartment 16 in asimilar fashion, as R_(STM):R_(SFM) (FIG. 4C). In exemplary embodimentsultra-congruent type prostheses, lateral sagittal conformity ratioR_(STM):R_(SFL) may be between 1.0 and 1.7, and medial sagittalconformity ratio R_(STM):R_(SFM) may be between 1.0 and 1.9, withlateral ratio R_(STL):R_(SFL) greater than medial ratio R_(STM):R_(SFM)by at least 0.2 through at least a portion of the flexion range. Inexemplary embodiments of posterior-stabilized type prostheses, lateralsagittal conformity ratio R_(STL):R_(SFL) may be between 1.4 and 1.8,and medial sagittal conformity ratio R_(STM):R_(SFM) may be between 1.0and 1.8, with lateral ratio R_(STL):R_(SFL) greater than medial ratioR_(STM):R_(SFM) by at least 0.4 through at least a portion of theflexion range. In exemplary embodiments of cruciate-retaining typeprostheses, lateral sagittal conformity ratio R_(STL):R_(SFL) may bebetween 1.1 and 2.6, and medial sagittal conformity ratioR_(STM):R_(SFM) may be between 1.1 and 2.2, with lateral ratioR_(STL):R_(SFL) greater than medial ratio R_(STM):R_(SFM) by at least0.5 through at least a portion of the flexion range.

Predicate devices have defined varying levels of medial and lateralconformity between the femoral condyles thereof and the correspondingtibial articular compartments. Generally speaking, in the case of tibialbearing component 12 and femoral component 20, the lateral conformity(defined by ratios R_(STL):R_(SFL) and R_(CTL): R_(CFL)) isapproximately equal to the lowest lateral conformity defined by thepredicate devices, while the medial conformity (defined by ratiosR_(STM):R_(SFM) and R_(CTM):R_(CFM)) is approximately equal to thehighest medial conformity defined by predicate devices.

5. Articular Features: Low Barrier to Femoral Rollback inPosterior/Lateral Articular Compartment.

As used herein, “jump height” refers to the proximal/distal distancethat a portion of femoral component 20 must traverse to sublux from thetibial bearing component 12. Referring to FIGS. 3A and 3B, medial andlateral articular compartments 16, 18 of tibial bearing component 12 areshown in cross-section to illustrate the location of distal-most points42, 44. The vertical distance between respective distal-most points 42,44 (FIGS. 3A, 3B) on the articular surface of tibial bearing component12 to the highest point at the edge of such articular surface is thejump height of tibial bearing component 12. Referring to FIG. 3A, medialfemoral condyle 22 (FIG. 2) would have to move proximally by a distanceH_(M) to move the contact point between condyle 22 and medialcompartment 16 from distal-most point 42 to the highest point along theposterior edge of medial compartment 16. For purposes of the presentdisclosure, such “highest point” is the point at which a posteriorextrapolation of medial articular track 26 reaches its proximal peak asthe extrapolated line advances toward the posterior edge of the tibialbearing periphery.

Thus, H_(M) may be referred to as the posterior jump height establishedby the particular curvature and geometry of medial articular compartment16. Jump height H_(M) is designed to provide an appropriately lowbarrier to desired posterior translation of the contact point betweenmedial condyle 22 and medial compartment 16 along medial articular track26, while also being sufficiently high to ensure that condyle 22 remainssafely engaged with articular compartment 16 throughout the range offlexion provided by the knee prosthesis.

Referring to FIG. 3B, lateral jump height HL is lower than medial jumpheight H_(M). Advantageously, setting H_(L) lower than H_(M) facilitatesfemoral rollback by presenting a relatively lower barrier to lateralcondyle 24 to traverse the posterior arcuate portion 30 of lateralarticular track 28 when the knee prosthesis is in deep flexion. In anexemplary embodiment, the height differential between lateral and medialjump heights H_(L), H_(M) are between 0.4 mm and 2.3 mm, which has beenfound to be an ideal range in order to facilitate femoral rollback whilemaintaining appropriate barrier to subluxation in both medial andlateral compartments 16, 18.

For example, FIG. 3C illustrates the height differential between jumpheights H_(L), H_(M) for eleven sizes of a posterior-stabilized tibialcomponent design in accordance with the present disclosure, when suchposterior-stabilized components are implanted with a tibial slope angleS (FIGS. 3A and 3B) of 3 degrees. As shown in FIG. 3C, the jump heightdifferential ranges from 1.15 mm in the smallest prosthesis size, thentrends generally downwardly to a minimum of 0.45 mm for the seventh of11 sizes. In other exemplary embodiments, the jump height differentialmay be as large as 2.68 mm. It is contemplated that a jump heightdifferential up to 3 mm may be used with prostheses according to thepresent disclosure.

FIG. 3D graphically depicts the jump height differentials between jumpheights H_(L), H_(M) for seven sizes of an ultra-congruent tibialcomponent design in accordance with the present disclosure, when suchultra-congruent components are implanted with a tibial slope angle S(FIGS. 3A and 3B) of 5 degrees. As illustrated, the jump heightdifferential ranges from 2.25 mm in the smallest prosthesis size, thentrends generally downwardly to a minimum of 0.56 mm for the largest ofthe seven sizes. By comparison, jump height differential for theabove-mentioned prior art high-flexion prosthesis, i.e., the ZimmerNatural Knee Flex Ultracongruent Tibial Bearing Component discussedabove, range from 0.09 mm to 0.39 mm. For non-high-flexion prior artdesigns, such as the Zimmer Natural Knee II Ultracongruent TibialBearing Component discussed above, the jump height differential rangesfrom 0.22 mm to 0.88 mm.

Similar to the trending of clocking angle α (FIG. 1A) described indetail above, a majority of prosthesis sizes represented by FIGS. 3C and3D experience a decrease in jump height differential from smaller tolarger sizes, and an overall substantial decrease is exhibited in thedifference between the smallest and largest sizes. Therefore, it may besaid that the trend in jump height differential for posterior-stabilizedand ultra-congruent tibial bearing components made in accordance withthe present disclosure is generally downward across the range of sizes.

6. Articular Features: Progressively Angled Posterior Spine Surface.

Turning now to FIG. 5A, spine 38 of tibial bearing component 12 definesposterior articular surface 48, which is designed to articulate withfemoral cam 40 (FIG. 2) of femoral component 20 during prosthesisarticulation, and particularly in mid- and deep flexion. As described indetail below, posterior articular surface 48 defines a progressivelyangled surface from a proximal, symmetric beginning to an angled distalend. This progressive angling accommodates external rotation of femoralcomponent 20 in deep flexion.

In use, initial contact line 50 represents the line of contact betweenfemoral cam 40 and posterior surface 48 when femoral cam 40 initiallycontacts spine 38 during flexion, while deep flexion contact line 52represents the line of contact therebetween when femoral cam 40 hasmoved posteriorly down posterior surface 48 to a deep flexionorientation. The total distance traversed by femoral cam 40 alongposterior surface 48 is referred to as the articular extent of posteriorsurface 48 as measured along a proximal/distal direction. In FIG. 5A,this articular extent may be represented as the distance from initialcontact line 50 to deep-flexion contact line 52. In an exemplaryembodiment, the articular extent of posterior surface 48 may be aslittle as 2 mm, 3 mm or 5 mm and as large as 10 mm, 15 mm or 20 mm, ormay be any value within any range defined by any of the foregoingvalues.

For purposes of the present disclosure, spine 38 is considered to bebisected by a sagittal plane into medial and lateral halves, such that aposterior spine centerline is formed along the intersection between thebisecting sagittal plane and posterior surface 48. Posterior surface 48defines a series of medial/lateral tangent lines, each of which istangent to posterior surface 48 at the spine centerline. For purposes ofillustration, a medial/lateral tangent line at the proximal end ofposterior articular surface 48 is illustrated as initial contact line 50in FIG. 5A, while a medial/lateral tangent line at the distal endthereof is illustrated as deep flexion contact line 52. In normalarticulation, initial contact line 50 will be coincident with theproximal-most medial/lateral tangent line and deep-flexion contact line52 will be coincident with the distal-most medial/lateral tangent line,as shown in FIG. 5A and described herein. However, it is appreciatedthat a certain amount of variation from the designed articular profileof a prosthesis is normal for in vivo prosthesis articulation.Therefore, the actual lines of contact between femoral cam 40 andposterior surface 48 during prosthesis use may deviate slightly from theintended medial/lateral tangent lines. For purposes of the presentdisclosure, prosthesis characteristics such as contact lines 50, 52 aredescribed solely in terms of the designed articular profile of theprosthesis when tibial and femoral components 12, 20 are articulatedthrough their nominal range of motion.

As illustrated in FIG. 5A, contact lines 50 and 52 are not parallel,with contact line 50 running medially/laterally along a directionparallel to a coronal plane, and contact line 52 oblique to the coronalplane such that line 52 advances posteriorly as it extends laterally(and, concomitantly, also advances anteriorly as it extends medially).Both of lines 50, 52 are parallel to the transverse plane, such that theangle formed between lines 50, 52 is solely with respect to the coronalplane. In an exemplary embodiment, the angle formed between initialcontact line 50 and deep-flexion contact line 52 may be as large as 3degrees. However, it is contemplated that other exemplary embodimentsmay form such angle at 7 degrees, and that an angle up to 10 degrees maybe used in some instances.

Turning to FIG. 5B, a cross-section of the medial portion of spine 38 isshown. Posterior articular surface 48 defines medial surface line 48A,extending between initial contact line 50 and deep flexion contact line52. As described in detail below, if posterior articular surface 48defined articular surface line 48A across the medial/lateral extent ofspine 38, spine 38 would be symmetric and external femoral rotation indeep flexion would not be accommodated in the manner provided by theasymmetric spine 38 of the present disclosure.

Turning to FIG. 5C, a cross-section medially/laterally bisecting spine38 is shown. Articular surface line 48B is defined by posteriorarticular surface 48 at this cross-section, and is shown juxtaposedagainst a hidden line representing articular surface line 48A from FIG.5B. As illustrated in FIG. 5C, lines 48A and 48B both extend from acommon proximal point along initial contact line 50. However, the distalpoint of line 48B (along deep flexion contact line 52) has movedposteriorly with respect to the distal end of line 48A. This posteriormovement reflects a progressively increasing material buildup along thebase or distal end of posterior articular surface 48, such that thisbase is increasingly “augmented” by additional spine material as thedeep flexion contact line 52 traverses from medial to lateral. Statedanother way, spine 38 is effectively thicker in the region of contactline 52 at the bisecting cross-section of FIG. 5C as compared to themedially-biased cross-section of FIG. 5B.

Turning to FIG. 5D, it can be seen that the process of materialthickening or augmentation described above with respect to FIG. 5C hasgrown and further intensified. Thus, while line 48C still originatesfrom a common proximal point with lines 48A, 48B along initial contactline 50, the distal end of line 48C along deep flexion contact line 52has moved further posteriorly with respect to line 48A. Thus, at thelateral edge of posterior articular surface 48, the base of spine 38 isthicker still.

In effect, the changing geometry of posterior articular surface 48 ofspine 38 from medial to lateral has the effect of imparting an angledappearance to the distal, deep-flexion portion of posterior articularsurface 48. The remainder of spine 38 is generally symmetrical about thesagittal plane, as illustrated in FIG. 5A. As femoral cam 40 traversesposterior articular surface 48 from the initial contact line 50 in midflexion to the deep flexion contact line 52 in deep flexion, the angleof the surface encountered by femoral cam 40 changes, thereby changingthe angle of the medial/lateral tangent lines described above withrespect to the coronal plane. In an exemplary embodiment, the initialtransition from non-angled contact lines (e.g., initial contact line 50)to angled contact lines (e.g., deep-flexion contact line 52) is spacedfrom a proximal terminus of posterior surface 48 by a distance ofbetween 0% and 100% of the total proximal/distal extent of posteriorarticular surface 48 (i.e., the transition may occur immediately or atthe very end of the flexion range, or anywhere in between). For purposesof the present disclosure, the proximal/distal extent of posteriorarticular surface 48 is the total distance traversed by femoral cam 40throughout the range of flexion motion. In the illustrative embodimentof FIG. 5A, this total proximal/distal articular extent of posteriorarticular surface 48 (i.e., the distance between a proximal start pointand a distal end point) may be as little as 2 mm, 3 mm or 4 mm and aslarge as 17 mm, 18.5 mm or 20 mm, or may be any value within any rangedefined by any of the foregoing values. The proximal end point coincideswith an initial contact between cam 40 and posterior articular surface48 at a prosthesis flexion of between 75 degrees flexion and 93 degreesflexion, while the distal end point is at a final contact between cam 40and posterior articular surface 48 at a prosthesis flexion of 155degrees.

Advantageously, the extent of the angling of posterior articular surface48 changes with changing levels of flexion. More particularly, the anglegrows by an amount corresponding to the expected increase in externalrotation of femoral component 20 as flexion progresses, thereby ensuringthat line contact is made between femoral cam 40 and posterior articularsurface 48 throughout the range of flexion of prosthesis 10. In anexemplary embodiment, a maximum external rotation of femoral component20 occur between 120 degrees flexion and 155 degrees flexion.

In contrast, if the posterior surface 48 of spine 38 had no angledsurface portions (i.e., if initial contact line 50 were parallel to deepflexion contact line 52) femoral cam 40 would transition from linecontact along initial contact line 50 to an increasingly point-likecontact near the medial edge of posterior articular surface 48.

In the exemplary embodiment illustrated in the figures, femoral cam 40is symmetrical in nature, such that accommodation of deep flexionexternal rotation without diminishment of cam/spine contact area isaccomplished solely through the above described lateral augmentation ofposterior articular surface 48 at the distal base of spine 38. Femoralcam 40 is described in detail in: U.S. Provisional Patent ApplicationSer. No. 61/561,658, filed on Nov. 18, 2011 and entitled FEMORALCOMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS(Attorney Docket: ZIM0915); U.S. Provisional Patent Application Ser. No.61/579,873, filed on Dec. 23, 2011 and entitled FEMORAL COMPONENT FOR AKNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS (AttorneyDocket: ZIM0915-01); U.S. Provisional Patent Application Ser. No.61/592,575 filed on Jan. 30, 2012 and entitled FEMORAL COMPONENT FOR AKNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS (AttorneyDocket: ZIM0915-02); U.S. Provisional Patent Application Ser. No.61/594,113 filed on Feb. 2, 2012 and entitled FEMORAL COMPONENT FOR AKNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS (AttorneyDocket: ZIM0915-03); and in U.S. Provisional Patent Application Ser. No.______, filed on even date herewith and entitled FEMORAL COMPONENT FOR AKNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS (AttorneyDocket: ZIM0915-04). The entire disclosures of each of theabove-identified patent applications are hereby expressly incorporatedherein by reference.

7. Articular Features: Posterior Eminence Providing Medial/LateralStability while Also Accommodating Hyperextension.

As noted above, FIGS. 6A and 6B illustrate an ultra congruent (UC) typetibial bearing component 112 designed for use with femoral component 120lacking the femoral cam 40 found on femoral component 20 (FIG. 2). Asalso noted above, ultra congruent tibial bearing components such ascomponent 112 lack spine 38 found on bearing component 12. Tibialbearing component 112 and femoral component 120 are otherwisesubstantially similar to tibial bearing component 12 and femoralcomponent 20 described above, with reference numerals of components 112and 120 analogous to the reference numerals used in components 12 and 20respectively, except with 100 added thereto. Structures of tibialbearing component 112 and femoral component 120 correspond to similarstructures denoted by corresponding reference numerals of tibial bearingcomponent 12 and femoral component 20, except as otherwise noted. In oneexemplary embodiment, femoral component 120 is similar or identical tocruciate-retaining (CR) femoral component 220 (FIGS. 4A and 4B).

In order to provide some medial/lateral constraint of femoral component20, particularly in extension and early flexion configurations,posterior eminence 138 may be provided. As shown in FIG. 6A, femoralcomponent 120 includes intercondylar notch 154 which, when in anextension orientation as shown, defines a width which provides minimalmedial lateral clearance with posterior eminence 138. Thus, any forcestending to urge femoral component 120 medially or laterally upon theproximal articular surface of tibial bearing component 112 encounterresistance as the inwardly facing lateral and medial sidewalls 155 _(L),155 _(M) of intercondylar notch 154 engage the lateral and medialsidewall portions 158 _(L), 158 _(H) of sidewall 158 of posterioreminence 138.

As best seen in FIG. 6A, anterior portion 158 _(A) of sidewall 158 ofposterior eminence 138 is generally arcuate and defines radius R_(A),thereby corresponding in shape to the inwardly facing anterior wall 155_(A) defining radius RNA which joins lateral and medial sidewalls 155_(L), 155 _(M) to form intercondylar notch 154. In an exemplaryembodiment, radius R_(EA) is defined at the outer periphery of proximalsurface 156. i.e., at the point where the planarity of proximal surface156 gives way to the distally sloping profile of sidewall 158.Similarly, radius R_(NA) of anterior wall 155 _(A) is measured at thatportion of anterior wall 155 _(A) which is complimentary to radiusR_(EA) when femoral component 120 is seated upon tibial bearingcomponent 112 in an extension orientation.

Thus, posterior eminence 138 and intercondylar notch 154 interfit withone another when femoral component 120 is in the extension orientationas shown. In an exemplary embodiment, radius R_(EA) may be 4 mm andradius R_(NA) may be 6 mm, such that a minimal clearance is providedbetween posterior eminence 138 and intercondylar notch 154 in the fullyextended position of FIG. 6A.

Further, as best seen in FIG. 6B, the transition from proximal surface156 to sidewall 158 is gradual and sloped, such that every potentiallyarticular portion of posterior eminence defines a radius of at least 1mm, including the sagittal/coronal radii R_(SC1), R_(SC2) defined bysidewall 158. Radii R_(SC1), R_(SC2) are shown denoted only in thesagittal perspective in FIG. 6D, it being understood that radii R_(SC1),R_(SC2) also extend around lateral and medial sidewall portions 158_(L), 158 _(M). Thus, radii R_(SC1), R_(SC2) extend around the medial,anterior and lateral portions of sidewall 158, thereby forming thegradual rounded transition between proximal surface 156 to thesurrounding articular surfaces of ultracongruent tibial bearingcomponent 112. Stated another way, any section plane perpendicular to atransverse plane (e.g., the transverse and coronal planes) taken throughany of lateral, medial and anterior sidewall portions 158 _(L), 158_(M), 158 _(A) of sidewall 158 will define radii greater than 1 mm atsuch sidewall portions 158 ₁, 158 _(M), 158 _(A), such as radii R_(SC1),R_(SC2). The posterior face of posterior eminence 138, which forms aportion of peripheral sidewall 172 of tibial bearing component 112, isnot designed for articulation with any structure as femoral component120 lacks any structure bridging the gap between medial and lateralcondyles 122, 124 (such as, for example, femoral cam 40 ofposterior-stabilized femoral component 20).

When femoral component 120 enters a hyperextension configuration (i.e.,when knee prosthesis 110 is articulated beyond full extension to a“backwards bend” of the knee), intercondylar notch 154 ascends theanterior portion of sidewall 158, gradually “beaching” or transitioninginto contact between the patello-femoral groove adjacent intercondylarnotch 154 and the medial and lateral portions of sidewall 158 overproximal surface 156. In an exemplary embodiment, such transition isdesigned to occur at 3.5 degrees of hyperextension (i.e., minus-3.5degrees flexion), though other exemplary embodiments may experience thetransition as high as 7 or 10 degrees of hyperextension. As shown inFIG. 6D, the level of hyperextension is controlled by the distancebetween anterior wall 155 _(A) of intercondylar notch 134 and anteriorportion 158 _(A) of sidewall 158 in extension (as shown in FIG. 6D).This distance can be made smaller for an earlier engagement and largerfor a later engagement.

The hyperextension “beaching” transition is further aided by thecomplementary angular arrangement of lateral and medial sidewalls 155_(L), 155 _(M) of intercondylar notch 154 as compared to lateral andmedial sidewall portions 158 _(L), 158 _(M) of posterior eminence 138.More particularly, FIG. 6A illustrates that angles μ_(F), μ_(T) areformed by sidewalls 155 _(L), 155 _(M) and 158 _(L), 158 _(M) ofintercondylar notch 154 and posterior eminence 138, respectively, andare both arranged to converge anterior of posterior eminence 138 asshown. In the illustrative embodiment of FIG. 6A, angles μ_(F), μ_(T)are measured in a transverse plane with femoral component 120 seatedupon tibial bearing component 112 in an extension orientation. Anglesμ_(F), μ_(T) are large enough to guide and center femoral component 120into engagement with posterior eminence 138 during hyperextension, butare small enough so that interaction between intercondylar notch 154 andposterior eminence 138 provides effective medial/lateral stability inextension and early flexion. In an exemplary embodiment, angle μ_(T), is21.5 degrees and angle μ_(F) ranges from 21 degrees to 23 degreesthrough a range of prosthesis sizes. However, it is contemplated thatangles μ_(F), μ_(T) would accomplish their dual rules of medial/lateralstability and hyperextension accommodation at any angle between 15degrees and 30 degrees.

The distal portion of the patellofemoral groove or sulcus, whichcoincides with and gradually transitions into the anterior terminus ofintercondylar notch 154, also has a shape which matches the profile oflateral and medial portions 158 _(L), 158 _(M) of sidewall 158.Advantageously, this matching shape and volume between intecondylarnotch 154 and posterior eminence 138 cooperates with the gently slopedsidewall 158 to accommodate hyperextension by minimizing the abruptnessof impact therebetween. Because hyperextension interaction is spreadover a large area, potential abrasion of posterior eminence 138 by suchinteraction is also minimized, thereby potentially extending the servicelife of posterior eminence 138 and, ultimately, of tibial bearingcomponent 112 in patients with hyperextending knees.

By contrast, the prior art Zimmer Natural Knee Flex Ultracongruent kneeprosthesis, available from Zimmer, Inc. of Warsaw, Ind, includes priorart tibial bearing component 112A having posterior eminence 138A havingareas which define a radius of less than 1 mm, as shown in FIG. 6E. Theangle formed between lateral and medial sidewall portions 158A_(L),158A_(M) of posterior eminence 138A is substantially less than angleμ_(T) defined by posterior eminence 138. More particularly, the priorart angle is 9-12 degrees, while angle μ_(T) is between 21 and 23degrees as noted above. Further, the intercondylar walls of the priorart femoral component designed for use with prior art tibial bearingcomponent 112A (not shown) has parallel intercondylar walls, i.e., noangle is formed between the intercondylar walls. Moreover, the distancebetween posterior eminence 138A and the anterior edge of theintercondylar notch of the prior art femoral component is larger thanthe corresponding distance defined by eminence 138 and anterior wall 155_(A) of the intercondylar notch of femoral component 120 (FIG. 6D), suchthat the prior art Zimmer Natural Knee Flex Ultracongruent kneeprosthesis lacks the capability for hyperextension “beaching” asdescribed above.

Turning back to FIG. 6C, medial/lateral stability is provided by thesloped surface provided by sidewall 158, and more particularly theheight H_(E) of proximal surface 156 over distal-most points 142, 144,of medial and lateral articular compartments 116, 118. However, suchstability is primarily desired for early flexion and is not needed indeeper levels of flexion. Accordingly, posterior eminence 138 is sizedand shaped to cooperate with intercondylar notch 154 to provide steadilydecreasing levels of medial/lateral constraint starting from a maximumat full extension and transition to a minimum at 90 degrees flexion,after which such constraint is no longer needed.

More particularly, as illustrated in FIG. 6A, lateral and medialsidewalls 155 _(L), 155 of intercondylar notch 154 diverge posteriorlyfrom the anterior terminus of notch 154 (at anterior wall 155A), suchthat the effective width between lateral and medial sidewalls 155 _(L),155 _(M) becomes steadily greater than posterior eminence 138 as flexionprogresses. Thus, additional medial/lateral space between posterioreminence 138 and intercondylar notch becomes available as prosthesis 110is transitioned into deeper flexion. An exemplary femoral component withsuch a divergent intercondylar notch is described in: U.S. ProvisionalPatent Application Ser. No. 61/561,658, filed on Nov. 18, 2011 andentitled FEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS (Attorney Docket: ZIM0915); U.S. Provisional PatentApplication Ser. No. 61/579,873, filed on Dec. 23, 2011 and entitledFEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS (Attorney Docket: ZIM0915-01); U.S. Provisional PatentApplication Ser. No. 61/592,575 filed on Jan. 30, 2012 and entitledFEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS (Attorney Docket: ZIM0915-02); U.S. Provisional PatentApplication Ser. No. 61/594,113 filed on Feb. 2, 2012 and entitledFEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS (Attorney Docket: ZIM0915-03); and in U.S. ProvisionalPatent Application Ser. No. ______, filed on even date herewith andentitled FEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS (Attorney Docket: ZIM0915-04). The entire disclosures ofeach of the above-identified patent applications are hereby expresslyincorporated herein by reference.

Posterior eminence 138 has a limited anterior/posterior extent whichalso operates to effect disengagement of posterior eminence 138 fromintercondylar notch 154 at a desired level of prosthesis flexion, asdescribed in detail below.

Thus, advantageously, posterior eminence 138 is shaped to cooperate withintercondylar notch 154 to be functional only where its medial/lateralstability function is desired, and to avoid interaction withintercondylar notch 154 where such function is no longer required. Ascompared to predicate posterior eminences, posterior eminence 138accomplishes this balance by having a rounded shape that iscomplementary to intercondylar notch 154 of femoral component 120 asdescribed above. For example, the prior art Natural Knee FlexUltracongruent knee prosthesis, available from Zimmer, Inc. of Warsaw,Ind., includes a tibial bearing component 112A (FIG. 6E) having aposterior eminence 138A which does not “interfit” with the correspondingfemoral component in the manner described above.

In the illustrated embodiment of FIG. 6C, proximal surface 156 issubstantially flat and/or planar and rises above distal-most points 144,142 by a height H_(E). In an exemplary embodiment, height H_(E) isbetween 3.8 mm and 5.5 mm. However, it is contemplated that height H_(E)may be as high as 10 mm, provided that anterior wall 155 _(A) isappropriately angled so as to prevent presentation of a non-rampedsurface to anterior portion 158 _(A) of sidewall 158 of femoralintercondylar notch 154 during hyperextension.

By contrast, a traditional “cruciate retaining” tibial bearing component212 (FIGS. 7A and 7B, described herein) includes intercompartmentaleminence 238 which defines a reduced height H_(E)′ and is not flat orplanar in its proximal surface. In an exemplary embodiment, heightH_(E)′ of intercompartmental eminence is between 3.7 mm and 5.2 mmacross a family of prosthesis sizes, but may have an alternative rangeof 2.0 mm-5.5 mm in some embodiments.

Further, posterior eminence 138 is distinguished from spine 38 ofposterior-stabilized tibial bearing component (FIG. 5A) in thatposterior eminence 138 is substantially shorter and defines a posteriorsurface that is non-articular. In an exemplary embodiment, for example,spine 38 protrudes proximally from the surrounding articular surface byat least 21 mm.

It is contemplated that posterior eminence 138 may define an increasedheight H_(E)″, and may include a rounded proximal surface 156′ withinthe scope of the present disclosure. More particularly, increased heightH_(E)″ and rounded proximal surface 156′ may be sized and shaped tomatch the distal end of the patellofemoral groove of femoral component120, such that sidewalls 158′ and proximal surface 156′ make continuouscontact around the adjacent periphery of the patellofemoral groove inhyperextension. Advantageously, this full-area contact may furtherreduce the contact pressures and impact magnitude experienced byposterior eminence 138 when femoral component 120 is hyperextended.

Posterior eminence 138 defines an anterior/posterior extent AP_(PE),which may be expressed in absolute terms or as a percentage of thecorresponding overall anterior/posterior extent AP_(UC) ofultracongruent tibial bearing component 112. For purposes of the presentdisclosure, anterior/posterior extent AP_(UC) is measured at the samemedial/lateral position as a sagittal plane bisecting posterior eminence138. Across an exemplary range of sizes of tibial bearing component 112,anterior/posterior extent AP_(PE) of posterior eminence 138 may be aslittle as 5 mm, 6 mm or 7 mm, and as much as 11 mm, 13 mm or 15 mm, ormay be any value within any range defined by any of the foregoingvalues. This range of anterior/posterior extents AP_(PE) correspond to arange of percentages of overall anterior/posterior extent AP_(UC) forthe respective sizes of tibial bearing component 112 that is as littleas 10% or 18.7% and as much as 20.5% or 30%, or any percentage withinany range defined by any of the foregoing values.

8. Soft Tissue Accommodation: Anterior/Lateral Relief Scallop.

Referring back to FIG. 7B, an anterior/lateral corner of tibial bearingcomponent 212 may have material removed near the proximal edge thereofto create scallop 268. Scallop 268 creates extra space for the adjacentiliotibial (IT) band, which could potentially impinge upon tibialbearing component 212 in some patients. In an exemplary embodiment,scallop 268 extends around the entirety of the anterior/lateral cornerof tibial bearing component 212. A detailed discussion of how theanterior/lateral corner of tibial prosthesis components are defined, andthe advantages of pulling such corners away from the bone periphery, maybe found in U.S. Patent Application Publication No. 2012/0022659 filedJul. 22, 2011 and entitled “ASYMMETRIC TIBIAL COMPONENTS FOR A KNEEPROSTHESIS” (Attorney Docket No. ZIM0815-01), the entire disclosure ofwhich is hereby expressly incorporated herein by reference.Advantageously, scallop 268 may be used in lieu of or in addition to ananterior/lateral pullback to avoid or minimize the impact of potentialimpingement of the iliotibial band on such comer.

Scallop 268 extends inwardly into the area of lateral articularcompartment 218, and downwardly toward the distal, baseplate-contactingsurface of tibial bearing component 212. Thus, scallop 268 is a chamferor fillet-like void in the periphery of tibial bearing component 212which creates a space that may be occupied by nearby soft tissues thatwould otherwise impinge upon such periphery. Scallop 268 may extenddistally almost to the distal baseplate-contacting surface, or mayextend a lesser amount distally. The inward (i.e., medial and posterior)extent of scallop into lateral articular compartment 218 may beapproximately equal to the distal extent, or may deviate from the distalextent. In an exemplary embodiment, scallop 268 occupies a 10-degreeangular sweep around the anterior/lateral portion of the periphery oflateral articular compartment 218.

It is also contemplated that similar scallops or relief spaces may beprovided around the periphery of tibial bearing component 212 toaccommodate other adjacent soft tissues, such as the medial collateralligament (MCL) and the lateral collateral ligament (LCL). Scallop 268and any other scallops positioned for relief around other soft tissuesare sufficiently sized and shaped to provide relief space for theintended soft tissue throughout a full range of flexion, and for a widevariety of patients.

9. Soft Tissue Accommodation: Anterior/Medial Bulbous Flare.

Referring now to FIGS. 8A and 8B, ultra-congruent type tibial bearingcomponent 112 is illustrated with a convex, bulbous flare 170 extendingoutwardly from peripheral sidewall 172. As described in detail below,flare 170 provides additional strength to medial compartment 116 at theanterior end thereof and protects adjacent soft tissues from abrasion,particularly the patellar tendon.

Most of sidewall 172 extends generally vertically (i.e., in aproximal-distal direction) between the distal, baseplate-contactingsurface 160 (FIG. 8B) and the proximal articular surfaces of tibialbearing component 112. Accordingly, a majority of the periphery ofbaseplate contacting surface 160 substantially fits within the proximalperiphery of the associated tibial baseplate (i.e., baseplate 14 shownin FIG. 1A). A detailed discussion of matching peripheries between atibial baseplate and associated tibial bearing component may be found inU.S. Patent Application Publication No. 201210022659 filed Jul. 22, 2011and entitled “ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS”(Attorney Docket ZIM0815-01), the entire disclosure of which is herebyexpressly incorporated herein by reference.

Additionally, most of the outer periphery of the proximal articularsurfaces of tibial bearing component 112 substantially matches thecorresponding outer periphery of the distal (i.e., baseplate contacting)surface 160. However, bulbous flare 170 extends beyond theanterior/medial periphery of baseplate contacting surface 160, andtherefore also extends beyond the corresponding periphery of theassociated tibial baseplate when tibial bearing component 112 is fixedthereto (such as is shown in FIG. 1A in the context of tibial bearingcomponent 12). Bulbous flare 170 thereby enables medial articularcompartment 116 to “overhang” or extend anteriorly and medially beyondthe periphery of tibial baseplate 14. Advantageously, this overhangallows an expanded anterior/medial and proximal reach of medialarticular compartment 116, while obviating the need for a larger tibialbaseplate. Avoiding the use of a larger baseplate size advantageouslyprevents overhang of tibial baseplate 14 over a small patient bone,while the bulbous flare 170 of tibial bearing component 112 preserves arelatively large articular surface. Accordingly, tibial componentsincorporating bulbous flare 170 are particularly suited to tibialprostheses for use in small stature patients, whose tibias commonlypresent a small proximal tibial resected surface which necessitates theuse of a correspondingly small tibial baseplate 14.

As shown in FIG. 8A, bulbous flare 170 includes a convex curvature whichextends up and around the proximal edge of medial articular compartment116. Advantageously, this convex profile and associated soft proximaledge presents only large-radius, “soft” edges to the patellar tendon,particularly in deep flexion prosthesis configurations. In one exemplaryembodiment, the convex curvature defined by bulbous flare 170 defines aflare radius R_(BF) (FIG. 8B) of at least 10 mm, which extends around apartially spherical surface. However, it is contemplated that bulbousflare 170 may also be formed as a complex shape incorporating multipleradii, such that bulbous flare 170 may be defined by any surface withconvexity in transverse and sagittal planes.

Referring now to FIG. 8A, another quantification for the broadly convex,soft-tissue friendly nature of flare 170 is the portion ofproximal/distal extent PD_(O) of the adjacent portion of sidewall 172that is occupied by proximal/distal extent PD_(F) of flare 170. In anexemplary embodiment, proximal/distal extent PD_(O) is the portion ofperipheral sidewall 172 of tibial bearing component not covered bytibial baseplate 14 when tibial bearing component 12 is assembledthereto, and proximal/distal extent PD_(F) of the convexity of flare 170occupies at least 80% of a proximal/distal extent PD_(O).

Also advantageously, the additional material afforded by bulbous flare170 at the anterior/medial portion of sidewall 172 provides a buttressfor the anterior edge of medial articular compartment 116, therebyenabling tibial bearing component 112 to readily absorb substantialanteriorly-directed forces applied by the femur during prosthesis use.

Yet another advantage provided by the increased size of medial articularcompartments 116 through use of flare 170 is that a larger femoralcomponent 120 may be used in conjunction with a given size of tibialprosthesis. For some patients, this larger femoral/smaller tibialprosthesis arrangement may provide a closer match to a healthy naturalknee configuration, and/or enhanced articulation characteristics.

Still another advantage to the convex, bulbous shape of flare 170 isthat the soft, rounded appearance thereof minimizes the visual impact ofan increased proximal height of medial articular compartment 116 and theincreased anterior extent thereof past the periphery of baseplatecontacting surface 160. This minimized visual impact allows sufficientlevels of buttressing material to be added to the anterior/medialportion of sidewall 172 while preserving surgeon confidence that theoverhang of flare 170 past baseplate contacting surface 160 isappropriate.

10. Bone Conservation and Component Modularity: Variable ComponentSurface Geometries.

As illustrated in FIG. 4A, medial and lateral articular compartments 16,18 of tibial bearing component 12 define substantially equal materialthicknesses between their respective superior, dished articular surfacesand opposing distal (i.e. inferior) surface 60. Stated another way, thecoronal “thickness profiles” of medial and lateral articularcompartments 16, 18 are substantial mirror images of one another about asagittal plane bisecting tibial bearing component 12.

For purposes of the present disclosure, a thickness profile of tibialbearing component 12 may be defined as the changing material thicknessesof medial and/or lateral articular compartments 16, 18 across a definedcross-sectional extent, such as an anterior/posterior extent in asagittal cross-section (FIGS. 9A-9D) or a medial/lateral extent in acoronal cross-section (FIGS. 10A-OC).

Thus, in addition to the coronal thickness profiles shown in FIG. 4A,medial and lateral articular compartments 16, 18 of tibial bearingcomponent 12 define sagittal thickness profiles (FIGS. 3A and 3B,respectively) between the superior dished articular surfaces of medialand lateral articular compartments 16, 18 and distal surface 60. Thesesagittal thickness profiles cooperate with anterior/posterior slope Sdefined by the proximal respective surface of tibia T (described indetail above) to define the anterior/posterior locations of medial andlateral distal-most points 42, 44, respectively. Thus, distal-mostpoints 42, 44 may shift anteriorly or posteriorly in response to achange in the sagittal thickness profile or tibial slope S, or both.

In alternative embodiments of tibial bearing component 12, showngenerally in FIGS. 9A-10C, the orientation of distal surface 60 withrespect to the superior articular surfaces of medial and lateralarticular compartments 16, 18 may be reconfigured. This reconfigurationalters the spatial relationship of distal surface 60 to the articularsurfaces, thereby effecting a change in the orientation of sucharticular surfaces with respect to the proximal resected surface oftibia T. As described below, this spatial alteration may be used tooffer alternative bearing component designs tailored to the specificneeds of some patients, while avoiding the need to recut or otherwisealter the geometry of the proximal tibia.

Referring now to FIG. 9A, one potential geometric reconfiguration oftibial bearing component 12 is alteration of the sagittal thicknessprofile to increase or decrease the anterior/posterior “tilt” of theproximal articular surfaces of medial and lateral articular compartments16, 18. For simplicity, only lateral articular compartment 18 is shownin FIGS. 9A-9D and described detail below, it being understood that asimilar geometric reconfiguration can be applied to medial compartment16 in a similar fashion.

For example, if a surgeon wishes to tilt tibial bearing component 12forward (such as to shift distal-most points 42, 44 anteriorly), he orshe may recut the proximal tibia to reduce tibial slope S. Similarly,increasing tibial slope S tilts tibial bearing component 12 backward andposteriorly shifts distal-most points 42, 44. However, a similar“tilting” of the tibial articular surface and shifting of sagittaldistal-most points, may be accomplished without altering tibial slope Sby using alternative tibial bearing components in accordance with thepresent disclosure, as described below. For example, where the superiorarticular surfaces of regular and alternative bearing components share acommon overall curvature and geometry, differing sagittal thicknessprofiles in the alternative component effects the same articular changesnormally achieved by a change in tibial slope S.

Referring to FIG. 9D, one exemplary alternative tibial bearing component312 is shown superimposed over tibial bearing component 12, with distalsurfaces 60 aligned such that changes to the articular surface oflateral articular compartment 18 are illustrated. Tibial bearingcomponent 312 features a sagittal radius R_(STL)′ defining radius centerC_(STL)′ which is anteriorly shifted along direction A with respect tosagittal radius R_(STL) and radius center C_(STL) of tibial bearingcomponent 12. This anterior shift reconfigures the spatial relationshipof the articular surface of lateral articular compartment 18 withrespect to distal surface 60. More particularly, this anterior shiftmimics a reduction in tibial slope S, because alternative lateralarticular compartment 18′ defines an articular surface which is“anteriorly tilted” so as to shift distal-most point 44 anteriorly tothe alternative distal-most point 44′, as shown in the dashed-linearticular surface profile of FIG. 9D. Conversely, center C_(STL) ofradius R_(STL) could be shifted posteriorly to mimic an increase inposterior slope S by causing a posterior shift of distal-most point 44.

When center C_(STL) is anteriorly shifted to alternative centerC_(STL)′, the resulting articular surface may not be identical to itsnon-shifted counterpart. However, the articular characteristics oftibial bearing components 12, 312 will be comparable, provided anoffsetting change in anterior slope S is made to place distal-mostpoints 44, 44′ at the same anterior/posterior position. Thus, a familyof tibial bearing components may be provided in which one component inthe family has an anteriorly shifted center C_(STL) as compared to theother component in the family. Depending on a surgeon's choice ofanterior slope S, the surgeon may intraoperatively choose from thefamily of components to accommodate the chosen slope S and place thedistal-most points of articular compartments 16, 18 at a desiredanterior/posterior location. To this end, components within the familymay have identical distal surfaces 60 such that each component in thefamily can be mounted to a common tibial baseplate 14.

Turning back to FIG. 9A, other alternative tibial bearing components312A. 312P are shown superimposed over tibial bearing component 12, witharticular compartment 18 aligned such that changes in distal surfaces60, 60A, 60P are illustrated. For example, bearing component 312Aselectively thickens portions of the sagittal thickness profile oflateral articular compartment 18, thereby angling the distal surfacethereof with respect to the superior articular surfaces. Alternativedistal surface 60A defines angle β_(A) with respect to distal surface 60of tibial bearing component 12. As compared with the unaltered bearingcomponent 12, bearing component 312A progressively adds material todistal surface 60 along a posterior-to-anterior direction, such aminimum amount of added material is present at the posterior-mostportion of distal surface 60 and a maximum amount of added material ispresent at the anterior-most portion of distal surface 60. However,alternative distal surface 60A is otherwise identical to distal surface60, such that either of distal surfaces 60, 60A can be mounted to thesame tibial baseplate.

Thus, the added material which defines distal surface 60A of tibialbearing component 312A operates in the manner of a wedge-shaped shimplaced between distal surface 60 and the adjacent superior surface 62 oftibial baseplate 14, except that the added material of component 312A isunitarily or monolithically formed therewith. As shown by a comparisonof FIGS. 9A and 9C, this wedge-shaped added material tilts the articularsurface of lateral articular compartment 18 posteriorly (i.e., theposterior portion of component 312A shifts distally relative to theanterior portion), thereby shifting distal-most point 44 posteriorly toalternative distal-most point 44A. As compared to bearing component 12,the magnitude of the posterior tilt (and therefore, of the posteriorlow-point shift) is controlled by increasing or decreasing angle β_(A)(FIG. 9A).

Conversely, tibial bearing component 312P (FIG. 9B) progressively addsmaterial along an anterior-to-posterior direction, thereby adding awedge-shaped portion of extra material to component 312P to definedistal surface 60P. Distal surface 60P is also identical to distalsurface 60, such that component 312P can be attached to tibial baseplate14. When so attached, the superior articular surface of lateralarticular compartment 18 is anteriorly tilted (i.e., the anteriorportion of component 312P shifts distally relative to the posteriorportion). As illustrated by a comparison of FIGS. 9A and 9B, distal-mostpoint 44 is shifted anteriorly to alternative distal-most point 44P. Ascompared to bearing component 12, the magnitude of the anterior tilt(and therefore, of the anterior low-point shift) is controlled byincreasing or decreasing angle β_(P) (FIG. 9A).

A similar selective thickening of tibial bearing component 12 may beemployed to provide alternative bearing components which allow a surgeonto intraoperatively correct for varus/valgus deformities. Referring nowto FIG. 10A, alternative tibial bearing components 412L, 412M definedistal surfaces 60L, 60M which progressively add material alongmedial-to-lateral and lateral-to-medial directions, respectively, ascompared to distal surface 60 of tibial bearing component 12. As withalternative surfaces 60A, 60P, distal surfaces 60L, 60M are otherwiseidentical to distal surface 60 such that any of components 12, 412M,412L can be mounted to a common tibial baseplate 14.

Distal surface 60L defines angle L with distal surface 60, effectivelyplacing the thickest part of a wedge-shaped shim of additional materialunderneath lateral articular compartment 18. Conversely, distal surface60M defines angle β_(M) with distal surface 60, such that the increasedthickness of the coronal cross-sectional profile is concentratedunderneath the medial articular compartment 16.

FIG. 10B illustrates tibial prosthesis 410L, which includes alternativetibial bearing component 412L having distal surface 60L mounted tosuperior surface 62 of tibial baseplate 14. Bearing component 412L isjuxtaposed the profile of tibial bearing component 12, which is shown indashed lines. As illustrated, the superior articular surfaces of medialand lateral articular compartments 16, 18 are tilted medially withrespect to the resected surface of tibia T (i.e., the medial portion ofcomponent 412L shifts distally relative to the lateral portion) whentibial bearing component 412L is attached to tibial baseplate 14.Bearing component 412L defining such a medial tilt may be employed, forexample, to intraoperatively correct for a varus deformity in the kneeof the patient without altering the geometry of the proximal tibial cutsurface or replacing tibial baseplate 14. The magnitude of the medialtilt is controlled by increasing or decreasing angle β_(L) (FIG. 10A).

Turning to FIG. 10C, another alternative tibial bearing component 412Mis shown juxtaposed against the dashed line profile of tibial bearingcomponent 12. Bearing component 412M is similar to component 412Ldiscussed above, except that distal surface 60M features a lateral tilt(i.e., the lateral portion of component 412M shifts distally relative tothe medial portion) when tibial bearing component 412M is attached totibial baseplate 14. Bearing component 412M defining such a lateral tiltmay be employed, for example, to intraoperatively correct for a valgusdeformity in the knee of the patient without altering the geometry ofthe proximal tibial cut surface or replacing tibial baseplate 14. Themagnitude of the lateral tilt is controlled by increasing or decreasingangle β_(M) (FIG. 10A).

In an exemplary embodiment, a set or family of tibial bearing componentsmay be provided which includes any combination of tibial bearingcomponents 12, 312A, 312P, 412M, and 412L. Further, multiple versions ofcomponents 312A, 312P, 412L, 412M may be provided, in which each versiondefines a unique value for angles β_(A), β_(P), β_(L), β_(M)respectively. When provided with such a family of components, a surgeonmay intraoperatively select a tibial bearing component which positionsdistal-most points 42, 44 at a desired location, and/or corrects forvarus or valgus deformities, without having to alter tibial slope S orchange tibial baseplate 14. In an exemplary embodiment, the geometry andcurvature of the superior dished articular surfaces of medial andlateral articular compartments 16, 18 will be identical for allcomponents provided in the kit, such that no other changes to thearticular characteristics of the tibial bearing component interminglewith the changes brought on by altering the thickness profile asdescribed above.

While the alternative tibial baseplates described above have eitherreconfigured sagittal thickness profiles or reconfigured coronalthickness profiles, it is contemplated that tibial bearing componentsmay be provided which incorporate reconfigurations to both the sagittaland coronal thickness profiles within a single tibial bearing component.Moreover, it is contemplated that any appropriate thickness profile orset of thickness profiles may be provided as required or desired for aparticular application.

Thus, a family of tibial bearing components provided in accordance withthe present disclosure obviates any need for a surgeon to recut theproximal surface of tibia T, and allows the surgeon to permanentlyimplant tibial baseplate 14 while also preserving the intraoperativeoption to 1) alter the anterior/posterior tilt of the articular surfacesof medial and lateral articular compartments 16, 18, and/or 2) alter themedial/lateral tilt or the articular surfaces, such as for correction ofa varus/valgus deformity.

Moreover, it is appreciated that a tibial bearing component inaccordance with the present disclosure may be provided in asingle-component design, i.e., not part of a kit, while still beingdesigned to “alter” the tilt of the superior articular surface. Forexample, the articular surface of an alternative bearing component maybe designed to may mimic the articular surface of a “regular” tibialbearing component (such as component 12, described above), even thoughthe two components are designed to cooperate with differinganteroposterior tibial slopes.

In some instances, for example, differing classes of tibial bearingcomponent (e.g., ultracongruent and posterior-stabilized) are designedto be used with differing tibial slopes. However, a surgeon may wish tointraoperatively select between these differing component classes, whichin turn may necessitate recutting of tibia T. However, in an exemplaryembodiment, ultracongruent tibial bearing component 112 (FIGS. 6Athrough 6C) may include distal surface 160 which defines ananterior/posterior slope with respect to medial and lateral articularcompartments 116, 118 which effectively “tilts” the articular surfacesthereof forward sufficiently to render ultracongruent tibial bearingcomponent 112 compatible with tibial slope S (shown in FIGS. 3A and 3Band described in detail above) used for posterior-stabilized tibialbearing component 12.

For example, an ultracongruent-type tibial bearing component may betypically designed for use with a tibial slope S equal to 3 degrees,while other bearing component designs (e.g., posterior-stabilizeddesigns) may use a 5 degree tibial slope S. In this situation,ultracongruent tibial bearing component 112 may be effectively “tiltedanteriorly” by 2 degrees in the manner described above, such that thearticular characteristics designed into the articular surfaces of tibialbearing component 112 are achievable with a 5-degree tibial slope S.Thus, a surgeon may make a proximal cut of tibia T to create ananteroposterior slope S of 5 degrees, for example, while achievingarticular characteristics normally associated with a tibial slope of 3degrees by implanting tibial bearing component 112 on tibial baseplate14. Thus, a surgeon may have the freedom to choose intraoperativelybetween ultracongruent tibial bearing component 112 and posteriorstabilized tibial bearing component 12 without having to alter tibialslope S or tibial baseplate 14.

Moreover, it is contemplated that changing thickness profiles or themoving the center of sagittal curvature of an articular surface asdescribed above may be accomplished with any combination ofcruciate-retaining, ultracongruent and/or posterior-stabilized designs.

While the present disclosure has been described as having exemplarydesigns, the present disclosure can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses or adaptations of the disclosureusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this disclosure pertains.

1. (canceled)
 2. A tibial bearing component for articulation with amedial femoral condyle and a lateral femoral condyle, the tibial bearingcomponent defining a tibial bearing component coordinate systemcomprising: a bearing component transverse plane extending along amedial/lateral direction and an anterior/posterior direction; a bearingcomponent coronal plane extending along a proximal/distal direction andthe medial/lateral direction, the bearing component coronal planeperpendicular to the bearing component transverse plane; and a bearingcomponent sagittal plane extending along the anterior/posteriordirection and the proximal/distal direction, the bearing componentsagittal plane perpendicular to the bearing component transverse planeand the bearing component coronal plane, the tibial bearing componentcomprising: an articular surface and an opposing distal surface, thedistal surface parallel to the bearing component transverse plane, thearticular surface including medial and lateral dished articularcompartments sized and shaped for articulation with the medial andlateral femoral condyles respectively, the medial and lateral dishedarticular compartments separated from one another by the bearingcomponent sagittal plane, the lateral articular compartment comprising aplurality of coronal cross-sectional profiles defining a lateral set ofcoronal distal-most points spanning a lateral anterior/posterior extent,the lateral set of coronal distal-most points defining a lateralarticular track, the lateral articular track having an anterior portionand a posterior portion, the posterior portion defining a curved linewhen projected onto the bearing component transverse plane.
 3. Thetibial bearing component of claim 2, wherein the anterior portiondefines a nominally straight line when projected onto the bearingcomponent transverse plane.
 4. The tibial bearing component of claim 3,wherein the medial articular compartment comprises a plurality ofcoronal cross-sectional profiles defining a medial set of coronaldistal-most points spanning a medial anterior/posterior extent, themedial set of coronal distal-most points defining a medial articulartrack, at least a portion of the medial articular track defining anominally straight line when projected onto the bearing componenttransverse plane.
 5. The tibial bearing component of claim 2, whereinthe lateral articular track defines a lateral sagittal distal-most pointalong the lateral anterior/posterior extent and the medial articulartrack defines a medial sagittal distal-most point along the medialanterior/posterior extent, the medial and lateral sagittal distal-mostpoints located in a common coronal plane, whereby the medial and lateralarticular tracks comprise respective distal-most points at a commonanterior/posterior location.
 6. The tibial bearing component of claim 2,wherein the tibial bearing component is implantable at ananteroposterior slope angle as measured in the bearing componentsagittal plane, the anteroposterior slope angle formed between thebearing component transverse plane and a transverse reference plane, thetransverse reference plane positioned to be normal to an anatomic axisof a tibia when the tibial bearing component is implanted, theanteroposterior slope angle having a positive value when an anterioredge of the tibial bearing component is elevated with respect to aposterior edge thereof, the lateral articular track defining a lateralsagittal distal-most point defined as a point among the lateral set ofcoronal distal-most points that is closest to the transverse referenceplane when the anteroposterior slope angle is equal to 5 degrees, thelateral sagittal distal-most point coincident with a posterior terminusof the anterior portion of the lateral articular track.
 7. The tibialbearing component of claim 2, wherein the curved line of the posteriorportion of the lateral articular track defines a radius having a radiuscenter, the radius center spaced medially from the lateral articulartrack, whereby the curved line arcs inwardly toward the medial articularcompartment.
 8. The tibial bearing component of claim 7, wherein themedial articular compartment comprises a plurality of coronalcross-sectional profiles defining a medial set of coronal distal-mostpoints spanning a medial anterior/posterior extent, the medial set ofcoronal distal-most points defining a medial articular track, the radiuscenter coincident with a projection of the medial articular track ontothe bearing component transverse plane.
 9. The tibial bearing componentof claim 8, wherein the medial articular compartment comprises aplurality of coronal cross-sectional profiles defining a medial set ofcoronal distal-most points spanning a medial anterior/posterior extent,the medial set of coronal distal-most points defining a medial articulartrack, the radius center coincident with a projection of the medialarticular track onto the bearing component transverse plane.
 10. Thetibial bearing component of claim 9, wherein the lateral articular trackdefines a lateral sagittal distal-most point along the lateralanterior/posterior extent, the lateral sagittal distal-most pointcoincident with a transition from the anterior portion to the posteriorportion of the lateral articular track.
 11. The tibial bearing componentof claim 10, wherein the medial and lateral sagittal distal-most pointsare located in a common coronal plane, whereby the medial and lateralarticular tracks comprise respective distal-most points at a commonanterior/posterior location.
 12. A tibial bearing component forarticulation with a medial femoral condyle and a lateral femoralcondyle, the tibial bearing component defining a tibial bearingcomponent coordinate system comprising: a bearing component transverseplane extending along a medial/lateral direction and ananterior/posterior direction; a bearing component coronal planeextending along a proximal/distal direction and the medial/lateraldirection, the bearing component coronal plane perpendicular to thebearing component transverse plane; and a bearing component sagittalplane extending along the anterior/posterior direction and theproximal/distal direction, the bearing component sagittal planeperpendicular to the bearing component transverse plane and the bearingcomponent coronal plane, the tibial bearing component comprising: anarticular surface and an opposing distal surface, the distal surfaceparallel to the bearing component transverse plane, the articularsurface including medial and lateral dished articular compartments sizedand shaped for articulation with the medial and lateral femoral condylesrespectively, the medial and lateral dished articular compartmentsseparated from one another by the bearing component sagittal plane, thelateral articular compartment comprising a plurality of coronalcross-sectional profiles defining a lateral set of coronal distal-mostpoints spanning a lateral anterior/posterior extent, the lateral set ofcoronal distal-most points defining a lateral articular track, thelateral articular track having an anterior portion and a posteriorportion, the posterior portion defining a curved line when projectedonto the bearing component transverse plane, wherein the lateralarticular compartment comprises an overall anterior/posterior span, andthe posterior portion of the lateral articular track occupies between20% and 50% of the overall anterior/posterior span.
 13. The tibialbearing component of claim 12, wherein the anterior portion defines anominally straight line when projected onto the bearing componenttransverse plane.
 14. The tibial bearing component of claim 13, whereinthe medial articular compartment comprises a plurality of coronalcross-sectional profiles defining a medial set of coronal distal-mostpoints spanning a medial anterior/posterior extent, the medial set ofcoronal distal-most points defining a medial articular track, at least aportion of the medial articular track defining a nominally straight linewhen projected onto the bearing component transverse plane.
 15. Thetibial bearing component of claim 12, wherein the lateral articulartrack defines a lateral sagittal distal-most point along the lateralanterior/posterior extent and the medial articular track defines amedial sagittal distal-most point along the medial anterior/posteriorextent, the medial and lateral sagittal distal-most points located in acommon coronal plane, whereby the medial and lateral articular trackscomprise respective distal-most points at a common anterior/posteriorlocation.
 16. The tibial bearing component of claim 12, wherein thetibial bearing component is implantable at an anteroposterior slopeangle as measured in the bearing component sagittal plane, theanteroposterior slope angle formed between the bearing componenttransverse plane and a transverse reference plane, the transversereference plane positioned to be normal to an anatomic axis of a tibiawhen the tibial bearing component is implanted, the anteroposteriorslope angle having a positive value when an anterior edge of the tibialbearing component is elevated with respect to a posterior edge thereof,the lateral articular track defining a lateral sagittal distal-mostpoint defined as a point among the lateral set of coronal distal-mostpoints that is closest to the transverse reference plane when theanteroposterior slope angle is equal to 5 degrees, the lateral sagittaldistal-most point coincident with a posterior terminus of the anteriorportion of the lateral articular track.
 17. The tibial bearing componentof claim 12, wherein the curved line of the posterior portion of thelateral articular track defines a radius having a radius center, theradius center spaced medially from the lateral articular track, wherebythe curved line arcs inwardly toward the medial articular compartment.18. The tibial bearing component of claim 17, wherein the medialarticular compartment comprises a plurality of coronal cross-sectionalprofiles defining a medial set of coronal distal-most points spanning amedial anterior/posterior extent, the medial set of coronal distal-mostpoints defining a medial articular track, the radius center coincidentwith a projection of the medial articular track onto the bearingcomponent transverse plane.
 19. A tibial bearing component forarticulation with a medial femoral condyle and a lateral femoralcondyle, the tibial bearing component defining a tibial bearingcomponent coordinate system comprising: a bearing component transverseplane extending along a medial/lateral direction and ananterior/posterior direction; a bearing component coronal planeextending along a proximal/distal direction and the medial/lateraldirection, the bearing component coronal plane perpendicular to thebearing component transverse plane; and a bearing component sagittalplane extending along the anterior/posterior direction and theproximal/distal direction, the bearing component sagittal planeperpendicular to the bearing component transverse plane and the bearingcomponent coronal plane, the tibial bearing component comprising: anarticular surface and an opposing distal surface, the distal surfaceparallel to the bearing component transverse plane, the articularsurface including medial and lateral dished articular compartments sizedand shaped for articulation with the medial and lateral femoral condylesrespectively, the medial and lateral dished articular compartmentsseparated from one another by the bearing component sagittal plane,wherein the and lateral dished articular compartments are asymmetric inshape relative to one another, the lateral articular compartmentcomprising a plurality of coronal cross-sectional profiles defining alateral set of coronal distal-most points spanning a lateralanterior/posterior extent, the lateral set of coronal distal-most pointsdefining a lateral articular track, the lateral articular track havingan anterior portion and a posterior portion, the posterior portiondefining a curved line when projected onto the bearing componenttransverse plane.
 20. The tibial bearing component of claim 19, whereinthe lateral articular compartment comprises an overallanterior/posterior span, and the posterior portion of the lateralarticular track occupies between 20% and 50% of the overallanterior/posterior span.
 21. The tibial bearing component of claim 19,wherein the anterior portion defines a nominally straight line whenprojected onto the bearing component transverse plane.